Fundamentals of
Chemical Engineering
Thermodynamics
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Fundamentals of
Chemical Engineering
Thermodynamics
Kevin D. Dahm
Rowan University
Donald P. Visco Jr.
University of Akron
Australia Brazil Japan Korea Mexico Singapore Spain United Kingdom United States
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Fundamentals Chemical Engineering
Thermodynamics
Kevin D. Dahm
Donald P. Visco, Jr.
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ISBN-13: 978-1-111-58070-4
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To Robin, my family, and all of my students and my
colleagues at Rowan.
Kevin D. Dahm
To my past, present and future students as well as my family, especially
Tracey, Mary, Matthew, and Lucy.
Donald P. Visco, Jr.
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Preface
“Thermodynamics is a funny subject. The first time you go through it, you don’t
understand it at all. The second time you go through it, you think you understand
it, except for one or two small points. The third time you go through it, you know
you don’t understand it, but by that time you are so used to it, so it doesn’t bother
you any more.”
Arnold Sommerfeld
M
ost likely, just about anyone who’s studied thermodynamics or taught it
can relate to the above quote. Though we were undergraduate students
a generation ago, we still remember how we, and many of our classmates,
perceived the subject of Chemical Engineering Thermodynamics when we first encountered it: complex and abstract, with tons of different equations and terms like
“entropy” and “fugacity” that were often hard to connect to anything that seemed
real (not to mention the symbols, with an array of carats, overbars, subscripts and
superscripts).
As teachers of the subject, we can’t shy away from its complexity—we have to
tackle it head on. What we can do is frame the subject in ways that make it more
accessible. The range of thermodynamics concepts and the long lists of equations
may always seem intimidating at first, but they needn’t seem arbitrary. Our goal with
this book is to provide a practical and relatable introduction for students who are
encountering Chemical Engineering Thermodynamics for the first time. This is an
important distinction, since several of the most popular books on the subject for this
course have also been used at the graduate level as well. By contrast, this book is
truly aimed at providing the “fundamentals” of chemical engineering thermodynamics for the undergraduate student. Once complete, the student will have the proper
background for follow-on undergraduate courses that rely on a solid foundation in
this field of study or for advanced courses in thermodynamics. In an effort to provide
this solid foundation in chemical engineering thermodynamics, we have incorporated several features into the book that are intended to make it more accessible to
a wide variety of learners:
Motivational Examples
Each chapter begins with a “Motivational Example” that introduces the topic of the
chapter and illustrates its importance. This is intended to benefit all learners, but
particularly global learners who require big picture insights to connect information,
and technical learners who require a practical application for everything.
Worked Examples
The book makes extensive use of examples in which the thought process behind the
solution is explained, step-by-step, and the practical significance of the material is
underscored. For some problems, an expanded version of the solution is available in
the students’ electronic supplements.
vii
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
viii
Preface
Detailed Derivations
We have made a big effort not to skip steps in the derivation of key concepts and
fundamental equations, instead taking an extra step or two such that the student
(who is new to the field) can follow the approach.
Margin Notes
The book makes extensive use of margin notes. These are intended to serve as a
“voice over the reader’s shoulder” guiding them through the book. Placing these
notes in the margins avoids interrupting the flow of the main text. The notes include
three recurring themes:
Margin Notes: These should be interpreted as an aside to the reader, providing
an interesting fact about the concept being presented or a quick digression on
the scientist or engineer associated with the development of that concept.
Margin Notes: Pitfall Prevention: This special type of margin note calls out to the
reader where, from our experience, common errors (both conceptual and from a
calculation standpoint) will normally occur.
Margin Notes: Food for Thought: These are special points that the reader might
consider in a deeper way related to the concept being presented. Some are simpler while others are more challenging. The student supplemental materials
include feedback on the Food for Thought questions.
Exercises and Problems
Each chapter ends with problems suitable for homework, which are divided into
“Exercises” and “Problems.” The Exercises are very focused and comparatively
short, and the answers are included in the students’ supplemental materials. Each exercise will test the student’s ability to apply one specific concept or perform one specific type of calculation, and the student can obtain immediate feedback on whether
he/she did it correctly. The Problems are longer and require synthesis of more concepts. Solutions to problems are available to the instructor both electronically and in
a printed solution manual. For many problems, the technology used to arrive at the
solutions (such as an Excel sheet) is provided for the instructor.
Organization
Each chapter is organized to be helpful to students with a variety of learning styles.
The introduction to each chapter includes a list of instructional objectives.
Each chapter closes with a bulleted summary that includes definitions of key
terms and highlights key concepts.
The book uses both inductive and deductive reasoning. In some places, key
equations and/or concepts are developed in the context of an example, and after
the example there is a discussion of how the concept generalizes. In other places
a more traditional deductive approach is used.
The book comes with a number of additional resources for both students and
instructors.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Preface
Instructor Resources
The supplemental material available to instructors includes:
An Instructor’s Solutions Manual, available in both print and electronic form.
Electronic resources used in the solutions of problems, such as an Excel sheet or
a PolyMath worksheet.
A Test Bank of suggested exam problems
Lecture Builder PowerPoint slides for each chapter
Student Resources
The electronic resources for students include:
Answers to Exercises
Feedback on Food for Thought questions
Links to experiential learning activities
MindTap Online Course and Reader
In addition to the print version, this textbook is also available online through MindTap, a personalized learning program. Students who purchase the MindTap version will have access to the book’s MindTap Reader and will be able to complete
homework and assessment material online, through their desktop, laptop, or iPad.
If your class is using a Learning Management System (such as Blackboard, Moodle,
or Angel) for tracking course content, assignments, and grading, you can seamlessly
access the MindTap suite of content and assessments for this course.
In MindTap, instructors can:
Personalize the Learning Path to match the course syllabus by rearranging content, hiding sections, or appending original material to the textbook content
Connect a Learning Management System portal to the online course and Reader
Customize online assessments and assignments
Track student progress and comprehension with the Progress app
Promote student engagement through interactivity and exercises
Additionally, students can listen to the text through ReadSpeaker, take notes and
highlight content for easy reference, and check their understanding of the material.
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ix
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Acknowledgments
The manuscript was peer-reviewed at three different stages of completion. The authors know this valuable feedback led to significant improvements in the book and
gratefully acknowledge the contributions of the following individuals who served as
reviewers:
D. Eric Aston
University of Idaho
Daniel H. Chen
Lamar University
Michael Mark Domach
Carnegie Mellon University
Isabel C. Escobar
The University of Toledo
Jeanette M. Garr
Youngstown State University
Douglas Ludlow
Missouri University of Science and
Technology
Edward Maginn
University of Notre Dame
Sohail Murad
University of Illinois at Chicago
William E. Mustain
University of Connecticut
Steven Nartker
Kettering University
Athanassios Panagiotopoulos
Princeton University
Ajit Sandana
University of Mississippi
Rafael Tadmor
Lamar University
John C. Telotte
Florida State University
Reginald P. T. Tomkins
New Jersey Institute of Technology
We’d also like to recognize a number of current and former students who made
substantial contributions to the book: Jason Giacomelli, Andrew Garrison, David T.
Hitchcock, William John Hoffman, Marc A. Izquierdo, Michele L. Marandola, Juan
Riveros, Christopher N. Robinson Zavala, Heather Malino, and Zachary Hinton all
participated through the Rowan University Junior/Senior Engineering Clinic, Sarah
E. Gettings and Chris Gies as summer interns at Rowan, Kate Clark through the
Williamstown High School Engineering Academy, and Keith McIver simply volunteered. These people reviewed the manuscript and provided insightful feedback and
drafted many of the figures, and the Engineering Clinic students contributed much
to the solutions manual. The book wouldn’t be what it is without their time and effort, and we greatly appreciate their contributions. We’d also like to thank Pamela
Kubinski, who collected the data used in Example 1-1, and Gina Tierno, who took
the photos in Figure 1-26. Ryan Pavlovsky, a student at Tennessee Tech University,
performed some research on systems that were important for Chapter 9, and we
gratefully acknowledge his efforts. Keith McIver contributed to the writing of questions and answers to the online tests for MindTap.
xi
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xii
Acknowledgments
I (KD) wish I could list all of the hundreds of people who have contributed to
my perspective on thermodynamics: authors, classmates and teachers, current and
former faculty colleagues. I’d also like to acknowledge the many teachers who have
helped me to become an effective writer. Of course, I have to thank all of the students who have shared a classroom with me over the years. Every semester as a
teacher I see new ideas, hear questions I’ve never heard before, and gain new insights. The book wouldn’t be what it is without all of these people. But most of all
I want to recognize my very first teachers, Arlene and Donald Dahm, and my wife
Robin for all of the help and support she’s given me throughout this project.
I (DV) would like acknowledge the efforts of those who have been important
to my personal education on the topic of thermodynamics, be it conversations with
colleagues, or textbook and journal authors that I have learned from. In particular
I would like to thank David Kofke, whose humility and insight have been inspirational. Finally, I would like to thank all of my past, current, and future students who
ask the question “why”, whether in class, after class, during office hours, or in emails.
When you ask “why” about a topic in thermodynamics, it challenges me to think
more deeply on the concept and, in turn, my comprehension of the subject becomes
that much deeper. I often tell students that thermodynamics is a beautiful and powerful subject, whose study helps us to a better understanding of why our world is the
way it is. And last, but certainly not least, I would like to thank my wife, Tracey, and
my children, Mary, Matthew, and Lucy. Many weekends, nights, and early mornings
were spent “working on the book”, and I have always appreciated their patience.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Contents
Preface vii
Acknowledgments xi
About the Authors xxi
Chapter 1 Introduction
1.1
1.2
1
The Role of Thermodynamics in Chemical Engineering 2
Motivational Example: The Conversion of Fuel into Electricity 3
1.2.1 Generation of Electricity 3
1.2.2 Forms of Energy and Energy Conversion 4
1.2.3 The Rankine Cycle 7
1.3
Systems and Processes 10
1.3.1 Fundamental Definitions for Describing Systems 13
1.3.2 Equilibrium and Steady State 16
1.4
The Forms of Energy 19
1.4.1 Force 19
1.4.2 Pressure 19
1.4.3 Work 22
1.4.4 Kinetic Energy 28
1.4.5 Potential Energy 29
1.4.6 Internal Energy 31
1.4.7 Heat 33
1.4.8 Temperature 35
1.4.9 Overview of the Forms of Energy 37
1.5
1.6
1.7
1.8
1.9
Summary of Chapter One 38
Exercises 39
Problems 40
Glossary of Symbols 42
References 43
Chapter 2 The Physical Properties of Pure Compounds 45
2.1
2.2
Motivational Example: Vapor Pressure of Water and Its Effect on the Rankine
Cycle 46
Physical Properties of Pure Chemical Compounds 47
2.2.1 The P-V-T Behavior of Real Compounds 47
2.2.2 Forms and Sources of Physical Property Data 51
2.2.3 State Properties and Path-Dependent Properties 52
xiii
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xiv
Contents
2.2.4 Intensive and Extensive Material Properties 54
2.2.5 State Properties of Multiphase Mixtures 56
2.3
Thermodynamic Models of Physical Properties 59
2.3.1 Enthalpy 59
2.3.2 Heat Capacity 60
2.3.3 Ideal Gases 64
2.3.4 Equations of State 72
2.3.5 Simple Models of Liquids and Solids 75
2.3.6 Summary of Simple Thermodynamic Models for Materials 76
2.4
2.5
2.6
2.7
2.8
Summary of Chapter Two 76
Exercises 77
Problems 78
Glossary of Symbols 81
References 81
Chapter 3 Material and Energy Balances 83
3.1
3.2
Motivational Example: Rockets
Material Balances 85
83
3.2.1 Mathematical Formulation of the Material Balance 86
3.2.2 Examples of Material Balances 87
3.3
3.4
3.5
3.6
Mathematical Expression of the First Law of Thermodynamics 93
Applications of the Generalized Energy Balance Equation 96
Combining the Energy Balance with Simple Thermodynamic Models 108
Energy Balances for Common Chemical Process Equipment 119
3.6.1 Valves 121
3.6.2 Nozzles 121
3.6.3 Pumps, Compressors, and Turbines 121
3.6.4 Heat Exchangers 123
3.7 Summary of Chapter Three 125
3.8 Exercises 125
3.9 Problems 126
3.10 Glossary of Symbols 129
3.11 References 130
Chapter 4 Entropy 131
4.1
4.2
4.3
Motivational Example: Turbines 131
Reversible Processes 134
Defining and Describing Entropy 141
4.3.1 Mathematical Definition of Entropy 142
4.3.2 Qualitative Perspectives on Entropy 146
4.3.3 Relating Entropy to Spontaneity and Directionality 147
4.3.4 Spontaneity and Reversibility 152
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Contents
4.3.5 Relating Entropy to Disorder 152
4.3.6 The Microscopic Definition of Entropy 154
4.4
The Entropy Balance 158
4.4.1 Applying the Entropy Balance to a Reversible Nozzle 160
4.4.2 Entropy and Efficiency 165
4.4.3 Unsteady-State Entropy Balances 170
4.5 The Carnot Heat Engine 177
4.6 Summary of Chapter Four 185
4.7 Exercises 186
4.8 Problems 186
4.9 Glossary of Symbols 190
4.10 References 190
Chapter 5 Thermodynamic Processes and Cycles 191
5.1
5.2
Motivational Example: Chemical Process Design 191
Real Heat Engines 194
5.2.1 Comparing and Contrasting the Rankine Cycle with the Carnot Cycle 194
5.2.2 Complete Design of a Rankine Heat Engine 197
5.2.3 Design Variations in the Rankine
Heat Engine 202
5.3
Refrigeration—The Vapor-Compression Cycle 208
5.3.1 A Household Refrigerator 208
5.3.2 Coefficient of Performance 213
5.3.3 The Carnot Refrigerator 213
5.3.4 Analyzing a Refrigeration Cycle with Simple Models 215
5.4
Liquefaction 219
5.4.1 Simple Liquefaction of Nitrogen 219
5.4.2 Energy-Efficient Compression Processes 221
5.4.3 Linde Liquefaction 224
5.5
5.6
5.7
5.8
5.9
Summary of Chapter Five 230
Exercises 231
Problems 232
Glossary of Symbols 236
References 236
Chapter 6 Thermodynamic Models of Real,
Pure Compounds 237
6.1
Motivational Example: Joule-Thomson Expansion 237
6.1.1 The Total Derivative 240
6.2
Mathematical Models of Thermodynamic Properties
245
6.2.1 The Triple Product Rule 246
6.2.2 Fundamental Property Relationships 247
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xv
xvi
Contents
6.2.3 The Expansion Rule 249
6.2.4 Maxwell’s Equations 250
6.2.5 Coefficient of Thermal Expansion and Isothermal Compressibility 252
6.2.6 Additional Applications of Thermodynamic Partial Derivatives 254
6.2.7 Summary of Useful Partial Differential Expressions 263
6.3
Heat Capacity and Residual Properties 264
6.3.1 Distinction between Real and Ideal Gas Heat Capacity 264
6.3.2 Motivation for the Residual Property 266
6.3.3 Definition of Residual Properties 270
6.3.4 Mathematical Expressions for Residual Properties 271
6.3.5 Summary of Residual Properties 273
6.3.6 Application of Residual Properties 274
6.4
6.5
6.6
6.7
6.8
Summary of Chapter Six 283
Exercises 284
Problems 284
Glossary of Symbols 287
Reference 287
Chapter 7 Equations of State (EOS) 289
7.1
7.2
Motivational Examples: Transportation of Natural Gas 289
Cubic Equations of State 295
7.2.1 The Rationale for Cubic Equations of State 295
7.2.2 Modern Cubic Equations of State 298
7.2.3 Solving Cubic Equations of State 299
7.2.4 Interpreting Solutions to Cubic Equations of State 302
7.2.5 Fitting Parameters to Cubic Equations of State 304
7.2.6 Vapor Pressure Curves and the Acentric Factor 310
7.2.7 Summary of Cubic Equations of State 312
7.2.8 Residual Properties from Cubic Equations of State 312
7.3
The Principle of Corresponding States 316
7.3.1 Illustrations of the Principle of Corresponding States 317
7.3.2 Generalized Correlations and Aggregated Data 324
7.3.3 Group Additivity Methods 330
7.4
Beyond the Cubic Equations of State 333
7.4.1 The Virial Equation of State 335
7.4.2 Microstates and Macrostates 338
7.4.3 Radial Distribution Functions 340
7.4.4 The Lennard-Jones Potential 344
7.5
7.6
7.7
7.8
7.9
Summary of Chapter Seven 347
Exercises 348
Problems 348
Glossary of Symbols 351
References 351
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Contents
Chapter 8 Modeling Phase Equilibrium
for Pure Components 353
8.1
8.2
Motivational Example: VLE Curves for Refrigerants 353
Mathematical Models of Phase Equilibrium 356
8.2.1 Qualitative Discussion of Phase Transitions 356
8.2.2 Mathematical Expression of the Equilibrium Criterion 357
8.2.3 Chemical Potential 359
8.2.4 The Clapeyron Equation 360
8.2.5 The Shortcut Equation 364
8.2.6 The Antoine Equation 368
8.3
Fugacity and Its Use in Modeling Phase Equilibrium 370
8.3.1 Calculating Changes in Gibbs Energy 371
8.3.2 Mathematical Definition of Fugacity 374
8.3.3 Poynting Method of Estimating Liquid and Solid Fugacity 381
8.4
8.5
8.6
8.7
8.8
Summary of Chapter Eight 388
Exercises 389
Problems 389
Glossary of Symbols 392
References 392
Chapter 9 An Introduction to Mixtures 393
9.1 Motivational Example: Mixing Chemicals—Intuition 393
9.2 Ideal Solutions 396
9.3 Properties of Mixing 400
9.4 Mathematical Framework for Solutions 407
9.5 Ideal Gas Mixtures 418
9.6 Summary of Chapter Nine 422
9.7 Exercises 423
9.8 Problems 423
9.9 Glossary of Symbols 426
9.10 References 427
Chapter 10 Vapor–Liquid Equilibrium 429
10.1 Motivational Example 429
10.2 Raoult’s Law and the Presentation of Data 431
10.2.1 Distribution Coefficients, Relative Volatility, and xy Diagrams 441
10.3 Mixture Critical Points 444
10.4 Lever Rule and the Flash Problem 447
10.4.1 Ternary Systems 457
10.5 Summary of Chapter Ten 461
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xvii
xviii Contents
10.6
10.7
10.8
10.9
Exercises 462
Problems 463
Glossary of Symbols 466
References 466
Chapter 11 Theories and Models for Vapor–Liquid Equilibrium of
Mixtures: Modif ied Raoult’s Law Approaches 469
11.1
11.2
11.3
11.4
Motivational Example 469
Phase Equilibrium for Mixtures 470
Fugacity in Mixtures 473
Gamma-Phi Modeling 478
11.4.1 Raoult’s Law Revisited 478
11.4.2 Henry’s Law 479
11.5 Modified Raoult’s Law 483
11.6 Excess Molar Gibbs Free Energy Models: An Introduction 485
11.7 Excess Molar Gibbs Free Energy Models: Usage 488
11.7.1 Temperature and Pressure Dependence of the Activity Coefficient 499
11.7.2 Excess Molar Gibbs Free Energy Models and the Flash Problem 502
11.8 Predictive Excess Molar Gibbs Free Energy Models 506
11.8.1 Van Laar Equation and Regular Solution Theory 507
11.9 Thermodynamic Consistency 516
11.9.1 Integral (Area) Test 517
11.9.2 Direct Test 521
11.10 Summary of Chapter Eleven 525
11.11 Exercises 526
11.12 Problems 527
11.13 Glossary of Symbols 532
11.14 References 533
Chapter 12 Theories and Models for Vapor–Liquid Equilibrium
of Mixtures: Using Equations of State 535
12.1 Motivational Example 535
12.2 Deviations from the Ideal Gas Model for the Vapor Phase 536
12.2.1 Mixture Fugacity Coefficients 536
12.2.2 Incorporating the Mixture Fugacity Coefficient 539
12.2.3 Gamma-Phi Modeling: Application Example 540
12.3 Phi-Phi Modeling 549
12.3.1 Equality of Mixture Fugacities 549
12.3.2 Mixture Fugacity Coefficients when Pressure Is a Dependent Variable 550
12.4 Ideal Solution for the Vapor Phase 560
12.5 Summary of Chapter Twelve 566
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Contents
12.6 Exercises 566
12.7 Problems 567
12.8 Glossary of Symbols 573
12.9 References 574
Chapter 13 Liquid–Liquid, Vapor–Liquid–Liquid, and
Solid–Liquid Equilibrium 575
13.1 Motivational Example 575
13.2 Liquid–Liquid Equilibrium 576
13.2.1 Impact of Pressure on Liquid–Liquid Equilibrium 579
13.2.2 LLE—Components of the Gibbs Free Energy 580
13.3
13.4
13.5
13.6
Various Types of LLE 581
Miscibility Gaps from a DG of Mixing Perspective 583
Stability Criterion for Liquid Mixtures 586
Modeling Liquid–Liquid Equilibrium 589
13.6.1 Modeling Liquid–Liquid Equilibrium—
Immiscible Systems 594
13.7 Vapor–Liquid–Liquid Equilibrium (VLLE) 595
13.8 Modeling of Vapor–Liquid–Liquid Equilibrium (VLLE) 599
13.9 Solid–Liquid Equilibrium (SLE) 601
13.10 Modeling Solid–Liquid Equilibrium (SLE) 604
13.10.1 Modeling Solid–Liquid Equilibrium (SLE): Simplifications 606
13.11 Summary of Chapter Thirteen 610
13.12 Exercises 611
13.13 Problems 614
13.14 Glossary of Symbols 616
13.15 References 616
Chapter 14 Fundamentals of Chemical Reaction Equilibrium 619
14.1 Motivational Example: Ethylene from Ethane 620
14.2 Chemical Reaction Stoichiometry 625
14.2.1 Extent of Reaction and Time-Independent Mole Balances 626
14.2.2 Extent of Reaction and Time-Dependent Material Balances 629
14.3 The Equilibrium Criterion Applied to a Chemical Reaction 630
14.3.1 The Equilibrium Constant 631
14.3.2 Accounting for the Effects of Pressure 635
14.3.3 Accounting for Changes in Temperature 650
14.3.4 Reference States and Nomenclature 659
14.4 Multiple Reaction Equilibrium 660
14.5 Summary of Chapter Fourteen 665
14.6 Exercises 665
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xix
xx
Contents
14.7 Problems 667
14.8 Glossary of Symbols 668
14.9 References 669
Chapter 15 Synthesis of Thermodynamic Principles 671
15.1 Motivational Example: Reactive Distillation 671
15.2 Energy Balances on Chemical Reactors 674
15.3 Simultaneous Reaction and Phase Equilibrium 682
15.4 A Complete Chemical Process 689
15.5 Summary of Chapter Fifteen 699
15.6 Problems 699
15.7 Glossary of Symbols 701
15.8 References 702
Appendix 703
Appendix A: Steam Tables 703
Appendix B: Mathematical Techniques 733
Appendix C: Physical Properties 735
Appendix D: Heat Capacity 739
Appendix E: Antoine Coefficients 741
Appendix F: Thermodynamic Diagrams 743
Appendix G: The Joback Group Additivity Method 748
Index 753
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About the Authors
Kevin D. Dahm joined the Rowan University Chemical Engineering department in 1999, and was promoted from Associate Professor to Professor in 2013. He
received his B.S. in Chemical Engineering from Worcester Polytechnic Institute in
1992 and his Ph.D. in Chemical Engineering from Massachusetts Institute of Technology in 1998 He has published over 30 journal articles, many of which are in the area
of engineering pedagogy, on topics such as instilling metacognition in engineering
students, pedagogically sound uses for process simulation, and assessment of student
learning. He has received four national awards from the American Society for Engineering Education: the 2002 ASEE PIC-lll Award, the 2003 Joseph J. Martin Award,
the 2004 Raymond Fahien Award, and the 2005 Corcoran Award. In addition, he
and his father Donald Dahm authored the book Interpreting Diffuse Reflectance and
Transmittance: A Theoretical Introduction to Absorption Spectroscopy of Scattering
Materials. Prior to joining Rowan University, he was a postdoctoral researcher at UC
Berkeley and an adjunct professor at North Carolina A&T State University.
Donald P. Visco, Jr.
is the Associate Dean for Undergraduate Studies and a Professor of Chemical & Biomolecular Engineering in the College of Engineering at the
University of Akron. He was previously employed at Tennessee Technological University. Prof. Visco’s research work focuses on molecular design and thermodynamic
modeling. He has won several awards for his research and educational activities,
including both the Dept. of Energy PECASE and the ASEE National Outstanding
Teaching Award. He has served as Chair of both the ASEE Chemical Engineering
Division as well as the Education Division of AIChE. Prof. Visco received both his
B.S. and Ph. D. degrees in Chemical Engineering from the University at Buffalo,
State University of New York.
xxi
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Fundamentals of
Chemical Engineering
Thermodynamics
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Introduction
1
LEARNINg OBJECTIVES
This chapter is intended to help you learn how to:
Define a system that has clearly understood boundaries
Define a process that has a clear beginning and end
Identify systems and processes that fall into specific categories, such as open, closed,
isothermal, and adiabatic
Define equilibrium and steady state, including how they are distinct from each other
Recognize systems and processes that are at equilibrium, at a steady state, or both
Recognize the forms in which energy can be stored by matter: internal energy, kinetic
energy, and potential energy
Recognize the forms in which energy can be transferred to or from matter: work and
heat
Quantify force, pressure, temperature, work, kinetic energy, and potential energy
T
hermodynamics is the study of energy, including the conversion of energy from one
form into another and the effects that adding or removing energy have on a system.
Thermodynamics is essential for the practice of chemical engineering. The principles
of thermodynamics have a fundamental role in how chemical processes are understood,
analyzed, and designed. This book is intended for readers who are being introduced to this
crucial subject for the first time.
The first chapter gives an overview of how and why thermodynamics is important and
introduces some fundamental concepts. In particular, two abilities that are foundational in
chemical engineering thermodynamics are
1. Recognizing the forms in which energy can be stored and transferred.
2. Identifying and analyzing systems.
Every chapter of this book opens with a list of objectives like the one above. Each
chapter also features a “motivational example,” where an examination of an engineering application underscores the significance of the topics presented. Normally, the motivational
example immediately follows the chapter instructional objectives, but in this chapter, the
motivational example is in Section 1. 2. First, we briefly explore why the field of thermodynamics as a whole is essential to the practice of chemical engineering.
1
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2
Fundamentals of Chemical Engineering Thermodynamics
1.1 The Role of Thermodynamics in Chemical Engineering
The number and variety of chemical products is staggering. Walking through the
aisles of a drug store or a supermarket, you will see hundreds of different products,
where chemical engineers played some role in the production of almost all of them.
Consider these examples:
Chemical engineers
are also concerned
with a wide variety of
by-products and waste
materials. The EPA’s
2010 Toxic Release
Inventory lists 593
individual chemicals
and 30 categories that
encompass dozens of
additional chemicals.
(This information is
available at: http://
www.epa.gov/tri/
trichemicals/index.htm)
The 2011–2013 Alfa Aesar® catalog contains a 2210-page alphabetical list
of the company’s chemical products, from Abietic Acid to Zirconium(IV)
1,1,1-trifluoro-2,4-pentanedionate.
The 2010 Physicians’ Desk Reference lists over 2400 prescription drugs sold in
the United States.
In a trip to a local supermarket, one of the authors counted over 200 different
household cleaning products, over 200 different hair care products, over 50 kinds
of toothpaste, and over 40 different insect repellents—not to mention the multitude of processed food products.
Many of these household products contain 10 or more ingredients, each of which
can itself be considered a chemical product.
These various products are manufactured in factories and chemical plants from
a wide variety of raw materials. Chemical engineers are responsible for designing
processes to manufacture them efficiently, economically, reliably, and (in particular)
safely.
Chemical engineering plays an important role in the manufacture of products
in a wide range of industries. Even across all of these different kinds of applications,
all chemical manufacturing processes basically follow a universal rule of thumb:
Chemical reactions convert raw materials (chemicals) into desired products (other
chemicals). From a business standpoint, the product must have a greater monetary
value than the raw materials being consumed (see Figure 1-1).
Recycle of unused A and B
Figure 1-1 shows
a single chemical
reaction, but many
chemical manufacturing processes require
multistep reactions
carried out in series of
reactors.
Figure 1-1 shows two
raw materials (A and
B), one product (P),
and one by-product
(U). Some chemical
processes will have
significantly larger
numbers of raw materials and by-products,
especially when multiple distinct reaction
steps are required to
make the product.
Raw material A
Chemical Reactions
A1B
P1U
Separation
Processes
Pure product P
Raw material B
Undesired by-product U
Figure 1‑1 A schematic of a typical chemical process.
As illustrated in Figure 1-1, chemical reactions almost never lead to products that
are pure. The mixture leaving a reactor normally contains not only the desired product but also by-products and/or unused raw materials. Consequently, some chemical
and/or physical process is needed to separate the substances that leave the reactor.
But what do the boxes labeled “Chemical Reactions” and “Separation Processes”
in Figure 1-1 actually represent? The specific processes depend on the answers to questions such as
How many different chemical reactions are necessary?
Can all reactions be carried out in a single reactor, or is a separate piece of
equipment needed for each reaction?
Figure 1-1 shows pure products leaving the Separation processes. How close to
‘pure’ is required?
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C H A P T E R 1 Introduction
How are the separations to be carried out?
Can the separations all be completed with a single piece of equipment, or are
several distinct separation steps needed?
At what temperature and pressure does each piece of equipment operate?
Answering questions like these is fundamental to the practice of chemical engineering, and the process designer will find that every process presents unique challenges and opportunities. Indeed, some products have more than one plausible route to
making them, each of which could have completely different answers to these questions.
Thermodynamics plays a vital role in the design of processes. The chemical engineer must consider thermodynamic properties when addressing questions such as
How much raw material and energy will it take to make 10 million pounds of
this product annually?
What methods can be used to separate this product from any by-products and
unused raw materials?
How much energy does it take to heat this process stream to the required temperature of 300°F?
How can the reactor conditions be optimized for the maximum production of
the desired product while minimizing the production of undesired by-products?
This book introduces and illustrates the principles of chemical engineering
thermodynamics by exploring the use of thermodynamics in the solution of engineering problems. Although focusing on examples of particular interest to chemical
engineers, the text also illustrates the extreme breadth of systems for which thermodynamics is applicable. Consequently, this book emphasizes examples drawn from
the design or components of chemical manufacturing processes, while also examining a broad range of engineering systems and problems. The next section considers an extremely broad and practical problem in our society: the generation of
electricity.
Typically, chemical
engineering programs
include a course on
chemical reactor
design and at least one
course on separations.
Distillation is the
most commonly used
separation technique,
though there are many
others.
Minimizing the
production of byproducts (U in
Figure 1-1) is a
major goal of green
engineering.
1.2 MOTIVATIONAL ExAMPLE: The Conversion of Fuel into Electricity
You might recognize the generation of electricity as an important engineering
problem, but not realize that electricity generation is also a chemical engineering
problem. And it’s true that the design of the electric generator itself falls more
within the sphere of electrical and mechanical engineering. Historically, however,
most electricity has come from burning fuels. A more global look at the problem of
obtaining electricity from fuel reveals a complex engineering challenge that is multidisciplinary in nature and in which chemical engineers have a clear role.
The cover of this book
shows an example of a
process that converts
fuel into electricity.
1.2.1 generation of Electricity
If you’ve taken a physics course that covered electricity and magnetism, you’ve
learned about induction of an electric current using a magnetic field. Consider the
conducting wire shown in Figure 1-2. The area surrounded by the wire is a flat,
round surface. Any closed circuit can be considered the boundary of a surface,
although the surface may not have a regular shape like that in Figure 1-2. If the
magnetic flux through the surface bound by the circuit changes, an electric current
is induced in the circuit. One strategy for inducing current is to create a magnetic
field with stationary magnets and to rotate the conductor within the field, as illustrated in Figure 1-3.
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3
4
Fundamentals of Chemical Engineering Thermodynamics
North
The use of electromagnetic principles
to generate electricity originated in the
1830s. (Cantor, 1996)
You may have seen
a children’s museum
exhibit in which you
powered a light bulb
by pedaling a bicycle,
and the bulb burned
brighter as you pedaled harder. Much of
the world’s electricity
is generated in exactly
this way, although
using more sophisticated equipment.
1
1
2
Figure 1‑2 A conducting
wire defining a circular surface.
2
South
Figure 1‑3 A conducting wire rotated in a stationary magnetic field induces a current.
The details of how an electric generator is designed are outside the scope of this
book. For our purpose, it is enough to know that rotary motion in the form of a spinning axle or shaft can be used to generate electricity. However, we remain faced with
the challenge: How do we turn the shaft?
1.2.2 Forms of Energy and Energy Conversion
Take a moment to make a list of sources of energy that are used in our society. Note
this doesn’t mean “batteries” and “wall sockets” but the root sources of the energy.
Your list might look something like this:
Coal
Oil
Natural gas
Sun
Wind
Running water
Lumber/plants
Nuclear sources
Geothermal sources
Herbert Hoover,
the 31st President
of the United States,
graduated with a
Geology degree from
Stanford University
and worked for many
years as a mining
engineer.
On this list, wind and water are of particular interest, because they are naturally
occurring fluids in motion and readily can be used to turn a shaft. Figure 1-4 shows
a variety of devices that harness wind and water power. They span the range from a
pinwheel, which is a simple child’s toy, to the Hoover Dam, which is one of the most
massive engineering projects ever undertaken. While widely different in scale, these
devices have much in common. They all have some type of fins or blades designed
to “catch” the motion of the wind or water. These blades are connected to a central
axle, which turns. In the language of thermodynamics, kinetic energy is converted into
shaft work.
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trekandshoot/Shutterstock.com
Carlos Caetano/Shutterstock.com
C H A P T E R 1 Introduction
Bryan Busovicki/Shutterstock.com
A pinwheel
Hoover Dam (outside)
visdia/Shutterstock.com
Hoover Dam (inside)
A waterwheel
MaxyM/Shutterstock.com
Geoffrey Kuchera/Shutterstock.com
Turbines in an offshore wind farm
A windmill
Figure 1‑4 Devices that harness wind and water power.
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5
6
Fundamentals of Chemical Engineering Thermodynamics
Kinetic energy and work are probably familiar concepts from physics. Briefly,
kinetic energy is energy of motion. Any object in motion has kinetic energy, which
is quantified by
K. E. 5
1
M 2
2
(1.1)
where M is the mass and v is the velocity of the object. Kinetic energy is discussed
in Section 1.4.4.
Work is a measure of energy transferred from one object to another. It is commonly quantified by
#
W 5 F dl
(1.2)
where l represents the linear distance travelled and F represents the force opposing
the motion. In devices like wind turbines, the motion is rotary—not linear—so shaft
work (WS) is quantified by
#
WS 5 d
In nuclear processes,
matter can be converted into energy.
This conversion is
described by Einstein’s
famous E = mc2 equation. Such processes
are not considered in
this book. Thus, we
model energy as a
“conserved quantity.”
Two fundamental
aspects of green engineering are obtaining
more energy from
renewable sources and
using energy (regardless of its source)
more efficiently.
(1.3)
where represents the angle through which the shaft turns and represents torque.
In Section 1.4.3, three different manifestations of work—flow work, expansion work,
and shaft work—are discussed.
Wind turbines and hydroelectric dams illustrate a crucial theme in thermodynamics: energy conversion.
Energy can be modeled as a “conserved quantity”; it can be converted from one form
into another but cannot be created or destroyed. This observation is known as the first
law of thermodynamics.
In thermodynamics, we are concerned with how energy is stored and how it is
transferred. Substances store energy in one of three forms: internal energy, potential
energy, and kinetic energy.
Kinetic energy is energy of motion.
Potential energy is energy of position. A stationary object 200 feet above the
ground is said to have “potential” energy because it can gain kinetic energy as
Earth’s gravity works on it.
Internal energy is energy stored within matter on a microscopic level. It is by far
the most important of the three in chemical engineering thermodynamics, but it is
less tangible than potential and kinetic energy. Temperature is a crucial metric for
quantifying how much internal energy a substance has, but one has to go down to
the molecular level to build a physical picture of what internal energy actually is.
Stored energy can be transferred in the sense that material can be moved from
location to location. Thus, if a tank is filled with water, all of the energy stored by the
water is now inside the tank. Energy can also be transferred in two additional forms:
work and heat.
Work is energy used to impart motion. It takes work to move a box, compress a gas, or turn a shaft. Motion occurs because the forces on an object are
imbalanced.
Heat is energy transferred without macroscopic motion. Heat transfer occurs
because of differences in temperature.
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C H A P T E R 1 Introduction
Section 1.4 discusses the various forms of energy in more detail. A major aspect
of engineering is designing machines and processes that harness the energy in natural resources and then convert that energy (as efficiently as possible) into the form
needed to accomplish desired tasks. The devices shown in Figure 1-4 effectively convert kinetic energy into shaft work. The spinning shaft can be used in turn to power
electric generators or any other task that requires shaft work; a classic application
for windmills and waterwheels is grinding grain into flour.
Return to the list of energy sources at the start of this section. Wind and running water represent naturally occurring kinetic energy that can be directly converted into work. Most of the energy sources on the list are fuels—coal, oil, gas,
and lumber—substances that have large amounts of internal energy. We know from
experience that this internal energy is readily converted into heat simply by burning
the fuel. The next section examines the challenge of converting this heat into work.
The same strategies can be applied when the heat comes from other sources,
such as sunlight or nuclear reactions.
1.2.3 The Rankine Cycle
The Rankine cycle converts heat into work. A schematic of the four steps of the
Rankine cycle—boiler, turbine, condenser, and pump—is shown in Figure 1-5. This
section explains the roles of these steps.
Wout
Boiler
The Rankine cycle is
named after William
John Macquorn
Rankine, a 19thcentury engineering
professor at the University of Glasgow, for
whom the Rankine
temperature scale is
also named.
Qin
Steam
turbine
Pump
Condenser
Win
Qout
Figure 1‑5 Schematic description of the Rankine cycle.
When burning fuel is used as the source of our energy, there is no naturally
occurring motion—we have to convert energy from some other form into kinetic
energy. A nozzle is a device that accelerates a fluid. Nozzles are used in rockets to
propel gases downward at a high velocity, causing the rocket to move upward due
to the conservation of momentum. As a more modest example, garden hoses have
nozzle attachments that produce faster-moving streams of water. (You may have
used your thumb as a sort of nozzle to influence the velocity of water coming from a
hose.) The kinetic energy in the exiting stream has to “come from” someplace, and
in fact, it comes from the fluid itself.
Figure 1-6 shows a schematic of a nozzle, where steam enters at P 5 5 bar and
T 5 450°C with negligible velocity and leaves at P 5 1 bar and T 5 228°C with a velocity of 669 m/s. The decreased temperature and increased velocity of the exiting fluid
represent a conversion of the steam’s internal energy into kinetic energy. (Example 4-6
demonstrates why the exiting stream has T 5 228°C and v 5 669 m/s, specifically.)
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7
Fundamentals of Chemical Engineering Thermodynamics
P 5 5 bar
T 5 4508C
v,0
P 5 1 bar
T 5 2288C
v 5 669 m/s
Figure 1‑6 A nozzle converts internal energy of a fluid
into kinetic energy; here steam is accelerated from a low
velocity to 669 meters per second.
Example 4-8 examines
the compression of a
gas and reveals that
the work required is
orders of magnitude
larger than in the
pump in Example 3-8.
A turbine (Figure 1-7) is an integrated device that in effect combines nozzles
with windmills. Consequently, a turbine is a machine that converts internal energy
into shaft work. The details of how nozzles and turbine blades are designed are
beyond the scope of this book; however, it must be noted that significant pressure
drops occur in a turbine, so the entering vapor or gas must be at a high pressure.
Furthermore, internal energy increases with increasing temperature, and at a given
Arogant/Shutterstock.com
8
(a) A steam turbine
Rotating axle 5
shaft work
High-pressure
steam entry
Low-pressure
steam output
(b)
Figure 1‑7 (a) Photograph of a large steam turbine, taken during
repair. (b) A schematic of a turbine.
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C H A P T E R 1 Introduction
set of conditions, vapors are higher in internal energy than liquids. Thus, in the
Rankine cycle, the stream entering the turbine is a high-temperature, high-pressure
vapor. In principle, the vapor can be any compound, but it is typically steam, which
is what we will assume here.
Where does the high-temperature, high-pressure steam come from? It takes a
great deal of shaft work to compress gases to higher pressures, because the changes
in volume accompanying pressure changes are so large. It would be self-defeating to
use up a lot of work to compress steam to a high pressure just so this steam could be
fed to a turbine to produce work. However, the volume of a liquid is much smaller
than the volume of an equivalent mass of vapor. As a result, compressing a liquid
requires significantly less work (often 2 to 3 orders of magnitude less) than compressing an equivalent mass of a vapor to the same pressure. Consequently, in the
Rankine cycle, a pump is used to compress liquid water to high pressure. This liquid
water then enters a boiler.
Figure 1-8 shows a schematic of a boiler in which fuel is burned in a furnace. The
liquid water enters, travels through coils inside the furnace, and emerges as water
vapor. In effect:
The internal energy in the fuel is converted into heat through a combustion
reaction.
The heat is transferred to the water and increases the internal energy of the water.
The increase in internal energy causes the water to increase in temperature, boil
into vapor, and possibly increase in temperature further after the phase change
is complete.
Sub-cooled or
saturated liquid
When describing a
pure compound, a
“saturated” liquid or
vapor is a pure liquid
or vapor at its boiling
point. This and other
terms describing the
state of a fluid are
defined in Section 2.2.1.
Coils, when compared
to straight pipes, provide more surface area
and produce more
effective heat transfer.
The typical chemical
engineering curriculum includes a heat
transfer course that
explores this in detail.
Saturated or
superheated vapor
Furnace
Figure 1‑8 Schematic of a boiler. In this case, the heat
required to boil the liquid comes from burning fuel.
Thus, low-pressure liquid water enters a series of three steps (pump, boiler, and
turbine, see Figure 1-5) and leaves as low-pressure steam. This device can be run by
continuously taking in fresh water and expelling steam, but that is inefficient and
impractical. For example, consider one of the early applications of the Rankine
cycle: the steam engine for trains. One cannot reasonably stop a train every ten
miles to take on fresh water. Instead, the steam leaving the turbine goes to another
unit where it gives off heat to the surroundings, condenses into liquid, and then goes
to the pump. This allows the Rankine cycle to operate at a steady state. A steadystate process is a continuous process in which all parameters are constant with respect to time. For example, if the material leaving the condenser is at T 5 100°C,
then a minute, an hour, or a day from now, it will still be at T 5 100°C if the process
is operating at a steady state.
Thus, the Rankine cycle is a continuously operating, closed loop in which water
sequentially circulates through the four steps—pump, boiler, turbine, condenser—
thereby converting heat into shaft work. We can now make several observations
The condenser in a
Rankine cycle is a
heat exchanger that
operates much like the
radiator of a car or the
coils on the back of a
refrigerator.
Steady state and equilibrium are discussed
in Section 1.3.2.
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9
10
Fundamentals of Chemical Engineering Thermodynamics
about chemical engineering and thermodynamics, and how these observations are
expressed in the Rankine cycle:
Energy can be regarded as a conserved quantity. It cannot be created or destroyed, but it can be converted from one form into another. This rule of experience is called the first law of thermodynamics and is discussed in a mathematical
sense in Section 3.3.
An integral part of engineering practice is the design of processes that accomplish an objective. In the Rankine cycle, the objective is the conversion of heat
into work.
Burning fuels is essentially conversion of the internal energy of the fuel into
heat. The heat can subsequently be converted into work using the Rankine cycle.
A chemical process typically is comprised of several steps called unit operations—
each of which accomplishes a specific task. The Rankine cycle includes four unit
operations: a pump, a turbine, and two heat exchangers (a boiler and a condenser).
Other examples of unit operations familiar to chemical engineers include reactors, compressors, and various separation processes such as flash drums (in which
liquid is partially vaporized due to a decrease in pressure), distillation columns
(which separate compounds through differences in their boiling points), absorbers (which use liquid solvents to dissolve gases), extractors (which use two immiscible liquid phases where solutes are purified through their attraction to one or
the other of these phases), and strippers (in which gas is bubbled through liquids,
evaporates volatile components of the liquid, and leaves non-volatiles behind).
Many chemical processes are designed to operate continuously—at a steady
state. Electrical generation, for example, needs to operate continuously, as there
is constant demand.
This section discussed the engineering challenge of generating electricity by
burning fuel, primarily in qualitative terms. Section 1.4 expands upon the concepts
of energy and energy conversion, laying a foundation for quantitative approaches to
thermodynamics problems. We will find that quantitative analyses cannot be carried
out without a very clear understanding of what specifically is being analyzed. Consequently, Section 1.3 introduces the concepts of a system and a process.
1.3 Systems and Processes
While billiard balls
are not a system of
practical interest to
chemical engineers,
the concept of work
is fundamental in
analyzing chemical
processes. Section 1.4.3
outlines three forms
of work that are of
primary significance in
chemical engineering.
Briefly, a system is a specific object or location that we are analyzing, and a process
is a specific time period or event that we are analyzing. We begin with an example.
The concept of work is most likely familiar from introductory physics courses but is
reviewed in Section 1.4.3. As stated mathematically in Equation 1.2, the work done is
equal to the distance moved multiplied by the force opposing the motion. Consider
the following problem:
Two billiard balls, a cue ball and an 8-ball, are initially at rest as shown in Figure 1-9a.
The cue ball is tapped, rolls two feet and strikes the 8-ball (Figure 1-9b). The collision
also causes the cue ball to slow down and change direction; it rolls another foot and
stops. The 8-ball moves two feet and stops. The final position of both balls is shown in
Figure 1-9c. The collisions are essentially frictionless, but the frictional force of the felt
surface on the rolling balls is 0.2 lbf . Find the work done.
We know some distances and the force of friction, so it seems like we should be
able to calculate work. The difficulty is “find the work done” isn’t a clear question.
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C H A P T E R 1 Introduction
11
2 ft.
1 ft.
2 ft.
2 ft.
Original
cue-ball
position
(a) Initial position
of billiard balls
Original
8-ball
position
(b) Collision
Original
cue-ball
position
(c) Final position
of billiard balls
Figure 1‑9 Position of cue ball (white) and 8-ball (black) (a) before the cue ball is
tapped, (b) at the moment of collision, and (c) after both come to rest.
Work was done on the cue ball when it was tapped. Work was done by the cue ball
on the 8-ball when they collided. Work occurred continuously as the balls rolled. We
could calculate any or all of these values of work. But which of these are we actually
interested in? Which is meant by “find the work done?”
A clearer question would be “How much work was done BY the 8-ball, BETWEEN
the time it was struck by the cue ball and the time it came to rest?” This is something we
can calculate immediately. The ball moved two feet, and the friction force opposing
the motion was a constant 0.2 lbf, so the work is (2 ft)(0.2 lbf) 5 0.4 ft-lbf.
In the language of thermodynamics, when we chose to examine the 8-ball, it
became our system. Furthermore, we defined a process, which has a clear beginning
(the moment the cue ball struck the 8-ball) and a clear end (the moment the ball
stopped rolling.) The location of the 8-ball in Figure 1-9a is the initial state of the
system, and the location of the ball in Figure 1-9c is the final state of the system.
A system is the specific portion of the universe that we are modeling. We will find
that the solution of practical problems is often much easier if we start by identifying
a specific system. In the Rankine cycle described in Figure 1-5, there are many different systems one might define:
The entire cycle.
Any single piece of equipment: the turbine, the pump, the boiler or the condenser.
Two sequential pieces of equipment, such as the pump and the boiler.
The furnace that supplies the heat to the boiler.
In a typical heat exchanger, there are at
least three different
systems: the side that
supplies the heat
(e.g., the furnace that
contains the burning
fuel), the side that
receives the heat (e.g.,
the pipe that carries
the water that is being
boiled), or the entire
exchanger. This is
discussed further in
Section 3.6.
Anything that has clear, unambiguous boundaries can be considered a system—
even if the boundaries are moving.
“The rocket and its contents” could be a system. Here the system would have
constant volume but would be moving as the rocket flies.
“The water in the pool” could be a system. The amount of water may be constantly changing through processes like rain and evaporation, but at any specific
time, the system has clear boundaries and a measurable amount of water in it.
In some applications, carefully defining a particular system might seem an unnecessary step; it might appear “obvious” what the system is. However, subtle errors
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12
Fundamentals of Chemical Engineering Thermodynamics
The magnitude of the
pump work in this
discussion is exaggerated to make a point.
We will find the pump
work is typically a
much smaller fraction
of the turbine work
than is indicated here.
are easy to make if one is not very precise or specific in defining an approach to a
problem. For example, referring to the Rankine cycle in Figure 1-10, consider the
question, “How much work is produced per minute?” Recognizing that the turbine
is the step that produces work, one might look at the turbine, see that it produces 500
kJ/min of work, and call that the answer. However, a more in-depth look at the process reveals that the pump requires work. If one must add 25 kJ/minute of work to
the pump for the process to function, then the process is generating 475 kJ/minute of
work overall. Careful consideration of how “the system” should be defined can help
prevent errors such as this one. Since the “net work” produced by the entire process
is a more meaningful number than the work from any one step, it makes sense to
start by defining the entire cycle as the system.
Wnet 5 2475 kJ/min
Many of the figures in
this book use dashed
lines to show the
boundaries of the
chosen system.
W 5 2500 kJ/min
Boiler
Qin
Steam
turbine
The sign of work is
negative when energy
leaves the system in
the form of work (as
in the turbine) and
positive when energy
is added to the system
in the form of work
(as in the pump). The
sign convention is discussed in Section 1.4.3.
Examples 3-6, 3-7,
and 5-9 are additional
problems in which
there is more than one
way the system could
reasonably be defined.
Thus, choosing the
most meaningful
system definition is
a step in solving the
problems.
A process in which
the state of the system is constant with
respect to time is
called a “steady-state
process”. Steady state
and equilibrium are
discussed in detail in
Section 1.3.2.
Pump
Condenser
Qout
W 5 125 kJ/min
Figure 1‑10 A sample Rankine cycle. If “the system” is the turbine, then
·
·
W = 2500 kJ/min. If “the system” is the pump, then W = 125 kJ/min.
·
If “the system” is the entire cycle, then Wnet5 2475 kJ/min.
Notice the use of dashed lines in Figure 1-10 to indicate the boundaries of three
possible systems; one contains the pump, one contains the turbine, and one contains
the entire heat engine. The control volume is a term used to describe the region of
space inside the boundaries of the system. Our primary interest is often in the material and energy crossing the boundaries of the system. The turbine is a good example.
In designing a heat engine, we are concerned with how much work is produced (this
is energy leaving the system), and we need to know how the turbine affects the material flowing through it, which can be understood by examining how the material
entering the system is different from the material leaving the system.
The specific events or period of time are called the process. Just as we need to be
clear and specific in defining the system, we also need to have a clear understanding
of when and how the process begins and ends. Example 1-6 will concern an object
that falls from a height of 50 feet and asks “What is the velocity when it hits the
ground?” Thus, to be useful, the process is defined as ending at the exact instant the
object strikes the ground. The language used throughout this book states
The system is the object itself.
The initial state of the system is that it is located at a height of 50 feet above the
ground and is stationary.
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C H A P T E R 1 Introduction
13
The process is everything that happens to the system as it transitions from
the initial state to the final state. Here, the process is very simple: The force
of gravity acts upon the object, causing it to fall. In more complex processes,
there could be several different events that affect the system, occurring either
consecutively or concurrently.
In the final state, the object is at a height of zero feet and has a non-zero downward velocity, which is the unknown we are trying to determine.
1.3.1 Fundamental Definitions for Describing Systems
This section introduces some terminology that is used throughout the book. We will
see, particularly in Chapters 3 and 4, that these are not merely semantic definitions.
The fact that a system is “open,” “closed,” or “adiabatic” has important implications
in how we build mathematical models of the system.
An open system can have both matter and energy entering and/or leaving the
system. A lake is an open system (see Figure 1-11); material can enter in the form of
rain and leave in the form of evaporation. In addition, rivers or streams might flow
into and out of the lake. Sunlight is an example of energy transfer, where it causes
the lake water to get warmer. Thus, the system can be affected even though no water
enters or leaves the system.
A closed system has no matter entering or leaving the system, although energy
transfer can still occur. Thus, a sealed aluminum can is a closed system (see
Figure 1-12); material cannot enter or leave the can, although its contents can be
heated or cooled.
Closed System
Open System
Evaporation
Precipitation
Solar energy
Heat transfer to
surroundings
Energy
Matter
Lake
Figure 1‑11 A lake is an example of an open system.
Figure 1‑12 A sealed can that has no
insulation is an example of a closed system.
The Rankine cycle described in Figures 1-5 and 1-10 provides examples of both
closed and open systems. If the turbine is defined as the system, it is an open system,
since there is steam continuously flowing both into and out of it. Similarly, the pump,
evaporator, and condenser are individual open systems. However, if one defines the
system to include all four unit operations as in Figure 1-10, then it is a closed system.
Water, as liquid or vapor, circulates continuously inside the system, but no material
crosses the boundaries of the system, and that fact makes it a closed system.
An adiabatic system has no heat entering or leaving the system. Note that
it’s possible for either closed or open systems to be adiabatic, as illustrated in
Figure 1-13. A section of pipe in a house may have water continuously flowing into
The Greek word
“adiabatos” means
“pathless” or “not
to be crossed.” In an
adiabatic system, heat
does not cross the
boundaries of the system; it takes no “path”
in or out.
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14
Fundamentals of Chemical Engineering Thermodynamics
Open Adiabatic System
Closed Adiabatic System
Heat
Heat
Matter
Matter
Figure 1‑13 An adiabatic system can be either an open or a closed system.
FOOD FOR
ThOUghT
1-1
What’s the point of
thinking about isolated
systems? If an isolated
system has no mass
transfer and no energy
transfer, how can anything happen to the
system?
The prefix “iso” means
“equal,” and the
Greek word “khoros”
means “space.” An
isochoric system has
constant volume—it
always contains the
same amount of space.
and out of it, making it an open system, but it also may be insulated enough to be
considered adiabatic. Note that if a system is described as adiabatic, this doesn’t exclude the possibility that work is added to or removed from the system.
An isolated system has neither matter nor energy entering or leaving the system.
If a cooler or thermos (shown in Figure 1-14) is closed and is so well insulated that it
can be modeled as perfectly insulated, then it is be considered an isolated system. A
system that is both closed and adiabatic can have work added or removed (e.g., the
“expansion work” described in Example 1-2), but an isolated system is not affected
by the surroundings in any way.
An isochoric system has a constant volume. Consider the water tank shown in
Figure 1-15. The tank has rigid walls, so if the tank is the system, it is isochoric. However, if the water in the tank is the system, its volume changes as the tank fills or
empties, so it is not isochoric.
Isolated System
Surroundings
FOOD FOR
ThOUghT
1-2
If your refrigerator has
a freezing compartment, try placing one
thermometer at the
wall that divides the
refrigerator compartment from the freezing compartment and
another thermometer
farther away from the
freezing compartment.
Are the temperatures
the same? If they are
one degree different,
is it still reasonable to
model the refrigerator
compartment as an
isothermal system?
Water in
Matter
Energy
Water
Water out
Figure 1‑14 A schematic of
an isolated system.
Figure 1‑15 Schematic of a water tank.
“The tank” is an isochoric system, but “the
water” is not necessarily isochoric.
An isothermal system has a constant temperature throughout the system. The
inside of a small refrigerator typically is regarded as an isothermal system. There is
probably little difference between the temperatures at different locations inside the
refrigerator. A three-story house usually can’t be regarded as isothermal, because
the top floor is often noticeably warmer than the bottom floor.
An isobaric system has a constant pressure throughout the system. Gaseous phases
can normally be modeled as isobaric systems, while liquids often cannot. Consider
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 1 Introduction
15
the swimming pool shown in Figure 1-16. The surface of the water is in contact with
the air, which exerts 1 atmosphere of pressure. At the bottom of the pool, the pressure is higher, because in addition to the atmospheric pressure, the force of gravity
on the water is also pushing downward. You’ve probably felt this effect while swimming or diving—the pressure increases as the depth of the water increases. If the
system were a glass of water, the pressure difference between the top and bottom of
the glass is small enough that it can be modeled as isobaric. For larger liquid systems,
pressure is not assumed to be uniform.
P 5 14.7 psia
9 ft
P 5 17.8 psia
Figure 1‑16 Pressure is a function of depth
in a swimming pool because of the force of
gravity pulling the water down.
The Greek word
“barrios” means
“weight.” Weight,
force, and pressure are
related (but not interchangeable) concepts.
Note that the terms “isothermal” and “isobaric” can cause confusion if not used
carefully, because they could reasonably be applied to either processes or systems.
The distinction is given here.
In an isothermal/isobaric system, the temperature/pressure is constant with
respect to position.
In an isothermal/isobaric process, the temperature/pressure is constant with
respect to time.
For example, if a can of soda is removed from a refrigerator and placed on a
counter, it gradually warms to room temperature, as illustrated in Figure 1-17. The
process is not isothermal, because the temperature of the soda changes with time.
But it is likely reasonable to say that the temperatures at all points in the soda can
are identical to each other at any particular time, so one can describe the system as
isothermal. In this book, in order to avoid such confusion, we only use the words
“isothermal” or “isobaric” when temperature or pressure is uniform with respect to
T 5 408F
T 5 708F
time passes
T 5 408F
T 5 708F
T 5 408F
T 5 708F
Temperature at different
locations at a given time
Figure 1‑17 A cold soda can is removed from a refrigerator and warms
to room temperature. Although the process is not isothermal (temperature
changes with time), the system has uniform temperature at any given time.
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16
Fundamentals of Chemical Engineering Thermodynamics
both position and time; that is, if both the system and the process are isothermal or
isobaric.
1.3.2 Equilibrium and Steady State
Equilibrium and steady state are two fundamental concepts that can be applied
to systems and processes. They are distinct from each other but are discussed together, because they are frequently confused with each other. This section defines
steady state and equilibrium and uses an example to illustrate the distinctions between them.
A system is at steady state when all properties of the system are constant with
respect to time.
A system is at equilibrium when there is no driving force for any change to its
state properties.
There are three possible “driving forces” that are important in chemical engineering thermodynamics.
FOOD FOR
ThOUghT
1-3
Look at the swimming
pool in Figure 1-16.
The pressure at the
bottom is higher
than the pressure
at the top. Is the
pool in mechanical
equilibrium?
A mixture of nitrogen and hydrogen
isn’t in equilibrium at
ambient T and P, but
the reaction forming
ammonia is so slow it
could be mistaken for
an equilibrium. While
a “chemical driving
force” for reaction
does indeed exist at
ambient temperature,
in practice high temperatures are needed
to produce significant
reaction rates.
1. Mechanical driving force: If the mechanical forces on an object are not balanced,
there is a driving force for acceleration. A falling object is at neither steady state
nor equilibrium. It’s not at steady state because the velocity and height are changing with time. It’s not in mechanical equilibrium because the downward force
(gravity) is stronger than the upward force (air resistance).
2. Thermal driving force: If two objects are at different temperatures, there is a
driving force for heat transfer, which leads to changes in the temperatures of
both objects. When cold water is mixed with hot water, heat flows from the hot
water to the cold water. The water is not at thermal equilibrium until it reaches a
uniform temperature.
3. Chemical driving force: Chemical reactions and phase changes are both chemical
processes that are of interest to chemical engineers, and a system is not in chemical equilibrium if there are driving forces for either or both processes present.
When pure nitrogen and pure hydrogen are brought together, there is a chemical
driving force for nitrogen and hydrogen to combine, forming the more stable
compound ammonia. If water at atmospheric pressure is placed in an environment where the temperature is 25°C, there is a chemical driving force for it to
freeze, because the solid phase is the more stable phase at this temperature and
pressure.
Thermal equilibrium exists when temperature is balanced throughout a system,
and mechanical equilibrium exists when force (or pressure) is balanced throughout a
system. Force, pressure, and temperature are described in more detail in Section 1.4.
The analogous property that we use to model chemical equilibrium is the fugacity.
Fugacity is an abstract, intangible property. It cannot be measured directly like temperature and pressure can; it only can be calculated using models that are introduced
in Chapter 8. We will revisit fugacity at that point.
It is important to recognize two facts:
1. A system can be at steady state without being at equilibrium.
2. A system can have state variables that change with position and still be at steady
state if the variables at all positions are constant with respect to time.
These points are illustrated by the following example.
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C H A P T E R 1 Introduction
A POT OF WATER ON A STOVE
17
examPle 1‑1
A covered pot of water is placed on a stove and the heat is turned on. The temperature
is monitored using thermocouples placed as shown in Figure 1-18. Predict what the temperature data will look like before reading on.
Defining the pot and its contents as the system, is the system at steady state? At
equilibrium?
T2
The cover on the
pot was an ordinary
lid, not sealed. Had
pressure in the pot
built up significantly
above ambient, the lid
would have popped
up and air would have
escaped. But this was
not observed to occur
significantly during the
experiment.
T3
T1
Water
Heat from Stove
Figure 1‑18 A covered pot of water, with three temperature sensors,
T1, T2, and T3.
SOLUTION: The specifics will vary based upon the size of the pot, the material from
which the pot is made, the amount of water, etc., but the behavior should be qualitatively
similar with any pot.
Initially, the temperature climbs at all three locations, as shown in Figure 1-19:
T1, the temperature inside the pot just above the liquid surface, climbs rapidly to
almost 100°C.
T2, the temperature at the base of the handle, climbs less rapidly and never
exceeds 75°C.
T3, the temperature at the end of the handle, increases only fractionally.
During this time, the system (the pot and its contents) is clearly not at a steady state.
Temperature of Pot (8C)
100
80
FOOD FOR
ThOUghT
1-4
60
T1
40
T2
20
0
0
10
40
20
30
Time Elapsed (min)
50
60
T3
How would you
expect these results
to be different if the
pot lid was sealed?
If the pot was well
insulated? If it was
both sealed and
insulated?
Figure 1‑19 Temperature measured at the locations shown in Figure 1-18.
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18
Fundamentals of Chemical Engineering Thermodynamics
While air is routinely
modeled as 79%
nitrogen and 21%
oxygen, it typically
contains traces of
many other chemicals.
Depending upon the
application, these
impurities may or may
not be important.
After approximately 15 minutes, the temperatures level off. The system is now at
steady state, because while the temperature is different at different locations, none of
these temperatures are changing with time, nor are any other properties changing with
time. The system, however, is not at equilibrium. There are temperature gradients and
therefore driving forces for heat transfer throughout the system. However, because the
heat transferred into each location is equal to the heat transferred out, the temperature
at each location is constant and there is a steady state.
While the system, as a whole, is not at equilibrium, some aspects of equilibrium
are evident in this system. The temperature of the water levels off at 100°C. At P 5 1
atm, the boiling point, T 5 100°C, is the only temperature at which pure liquid water
and pure water vapor can exist in equilibrium with each other. Here, the vapor phase
doesn’t actually contain pure water vapor; it contains a mixture of air and water vapor.
The topic of transitions between phases, and how they relate to equilibrium, is introduced in Chapter 2 for pure compounds. Discussion of mixtures begins in Chapter 9.
For now, we will simply note that if “the system” had been defined as a small region
of space containing the surface of the liquid, we could likely say this system was at
equilibrium:
The water at the surface and the air immediately above the surface are both at
P 5 1 atm.
The water at the surface and the air immediately above the surface are both at
T 5 100°C.
The water at the surface and the water vapor in the air immediately above the surface are in chemical equilibrium; though we won’t be discussing how to model chemical equilibrium mathematically until Chapter 8.
Figure 1-19 shows
the top of the pot
was below 80°C.
Consequently the
water boiled, rose
to the top as vapor,
condensed on the lid,
and fell back down.
Chemical processes
that are designed to
operate at steady state
can run continuously
for weeks or months
at a time.
But when we define the system as “the pot and its contents,” we see significant temperature gradients in this larger system; the entire system is not at equilibrium.
Note that if the pot didn’t have a lid, the system would be at neither steady state nor
equilibrium. Vapor is continuously formed from the boiling liquid. Without the lid, this
water vapor would escape. Thus, the amount of water in the pot would decrease with
time, and if any property of the system is changing with time, then the system is not at
steady state. However, with the lid in place, the contents of the pot are a closed system;
liquid water turns into vapor, and vapor turns back into liquid, but the total amount of
water in the system is constant.
Example 1-1 examined a real system—a pot of water on a stove—for a period of
one hour. Figure 1-19 illustrates a period of time during which the system was not at
steady state, and a point after which steady state was maintained for the rest of the
hour, but there was never an equilibrium state.
We said in Section 1.3 that a thermodynamic process needs to have a clear beginning and end. If a system is at steady state, identifying a beginning or end to
the process isn’t necessary, since a steady state can last indefinitely. Consider the
time period from 20 to 60 minutes in Figure 1-19; temperatures T1, T2, and T3 were
constant during this time and would have remained constant until some change
disturbed the system, such as the burner being turned off, or the lid being removed
from the pot.
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C H A P T E R 1 Introduction
19
1.4 The Forms of Energy
Here we formally state the first law of thermodynamics, which was alluded to
previously:
The first law of thermodynamics states that energy is a conserved quantity. Energy
can be converted from one form into another, and it can be transferred from one location to another, but it cannot be created or destroyed.
This fundamental principle can be applied to a huge range of practical engineering problems, but it begs the question, “what precisely is energy?”
This section describes three forms in which energy can be stored in a material:
internal energy, potential energy, and kinetic energy. It also describes two forms in
which energy can be transferred from one location to another: heat and work. We
start with work because it allows us to create a framework for all of the others.
Briefly, when an object is moved, “work” can be regarded as the amount of energy
required to move it. Work is defined formally and mathematically in Section 1.4.3.
The potential to do work is what makes the other forms of energy also recognizable
as energy.
We begin with a discussion of force and pressure, which are not forms of energy
but are foundational principles in the analysis of work.
1.4.1 Force
Newton’s second law of motion quantifies the force (F) acting on an object as the
product of the mass (M) of the object times the acceleration (a) resulting from the
force:
F 5 Ma
(1.4)
Gravity is the most familiar force in our everyday lives, and an object’s acceleration due to gravity is given the symbol g. On the surface of the Earth, g 5 9.8066 m/s2
(also 32.174 ft/s2).
When SI units are used in Equation 1.4, the right-hand side will have units of
kg · m/s2. A Newton is defined as the force required to accelerate 1 kg by 1 m/s2; thus,
1 N 5 1 kg · m/s2. This conversion factor is used so routinely that in some texts it is
written into Equation 1.5 explicitly, as
F5
1
ma
gc
(1.5)
where gc 5 1 kg · m · s22 · N21.
In English units, a pound-force (lbf) is the force required to accelerate 1 poundmass (lbm) by 32.174 ft/s2. Thus, a pound-force is equivalent to the force exerted by the
earth’s gravity on a 1 pound-mass object or, numerically, as 1 lbf 5 32.174 lbm · ft · s22.
FOOD FOR
ThOUghT
1-5
You undoubtedly
have an intuitive
notion of what energy
is, but how would
you define “energy”,
exactly? Consider this
before reading further.
Don’t be confused if
one of your textbooks
says F = ma and
another book says
F = ma/gc. All books
agree that 1 Newton
equals 1 kg · m/s2; it
doesn’t much matter
whether we give this
unit conversion factor
a name (gc ) or not.
Recall from physics
that acceleration is the
first derivative of
velocity, and velocity
is the first derivative of position. Both
derivatives are taken
with respect to time.
1.4.2 Pressure
Pressure (P) is defined as the force (F ) acting on a surface, divided by the area (A)
of the surface upon which the force is acting:
P5
F
A
(1.6)
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20
Fundamentals of Chemical Engineering Thermodynamics
FOOD FOR
ThOUghT
1-6
Can you think of a
situation in which a
system expands, but
there is no force opposing the motion?
In many engines, cylinders have openings
that are covered or
uncovered depending upon the position
of the piston. Thus
the piston can also
be made to act like a
valve.
examPle 1‑2
The force opposing
the motion is used
in calculating work.
This is discussed in
Section 1.4.3.
The SI unit of pressure is the Pascal, defined as the pressure resulting from one
Newton of force acting on an area of 1 m2. Another commonly used pressure unit is
psi, which stands for pounds-force per square inch. At sea level, atmospheric pressure is 101.325 kPa (14.696 psi). The bar is another unit of pressure, defined as 100
kPa. This is a convenient unit, because 1 bar is approximately 1 atm, and is used
frequently in this book.
In thermodynamics, one physical phenomenon we encounter frequently is
expansion (and contraction). When something expands, there is generally a force
opposing the expansion.
Figure 1-20, in Example 1-2, shows a type of system considered many times in this
book: a gas confined in a piston-cylinder device. A piston is a moving seal that confines a gas in a cylinder while allowing it to expand or contract. In a typical automobile
engine, the piston is attached to a crankshaft. The release of heat when fuel is combusted leads to rapid expansion of gases, and the piston and crankshaft transfer the
work from this expansion into power to move the vehicle. The piston in Example 1-2
isn’t attached to anything; it has no purpose other than to confine the gas while
allowing the volume to change. This isn’t a particularly practical system, but it is used
routinely in thermodynamics and throughout this book because it is a simple system
that is relatively easy to picture, and it provides a useful way to illustrate concepts.
gAS IN A PISTON-CyLINDER DEVICE
A gas is confined in a cylinder that is 5 cm in diameter with a movable piston at the top,
as in Figure 1-20. The piston itself weighs 0.1 kilograms. If the gas is heated and expands,
what is the force opposing the expansion? Assume friction is negligible.
Initial State
Patm 5 101.3 kPa
Final State
Patm 5 101.3 kPa
Patm
Patm
M 5 0.1 kg
M 5 0.1 kg
Pgas
Pgas
5 cm
FOOD FOR
ThOUghT
1-7
How would the problem be affected if the
cylinder were oriented
sideways instead of
vertically?
5 cm
Figure 1‑20 Expansion of a gas in a piston-cylinder device.
SOLUTION:
Step 1 Define the system
Define the gas as the system. Why choose the gas rather than the piston? Because we
are trying to find the force opposing the expansion of the gas; if we define the gas as
the system, then the force we’re trying to find is an external force that is acting on the
system.
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C H A P T E R 1 Introduction
Step 2 Identify forces opposing the motion
The piston moves upward as the gas expands, so the force of gravity on the piston is opposing the motion. In addition, the atmospheric pressure, which is pushing down on the
top of the piston, is also opposing the motion. Conceptually, friction would be an additional force opposing the motion, but we are told friction is negligible in this case.
Ftotal 5 Fpiston 1 Fatmosphere
21
If the piston were
attached to something,
like a crankshaft, then
that could contribute
additional forces
opposing the motion.
(1.7)
Step 3 Quantify the force of gravity on the piston
Force due to gravity is calculated as
F 5 Mg
(1.8)
So for a 0.1 kg piston, this force is
1
Fpiston 5 s0.1 kgd 9.8
m
s2
21
1N
kg ? m
1
s2
2
5 0.98 N
(1.9)
Step 4 Quantify the force of the atmosphere on the piston
Pressure was previously defined as
P5
F
A
(1.10)
The force F exerted by the atmosphere is what we want to determine. Thus,
Fatmosphere 5 PA
(1.11)
Since nothing unusual is stated about the location of the cylinder, we can assume the
atmospheric pressure is P 5 101.3 kPa. A can be found from the known piston diameter
of 5 cm:
Fatmosphere 5 s101.3 kPad
1 4 2 s5 cmd
2
(1.12)
The area of a circle is
πd 2y4.
Recall that 1 Pascal is defined as 1 N/m2; introducing this definition and converting units gives
1
1000 Pa
Fatmosphere 5 s101.3 kPad
kPa
21
21 2
N
m2
Pa
1
1
2
1m 2
s5 cmd2
5 198.9 N
4
100 cm
Step 5 Find total pressure
Ftotal 5 Fpiston 1 Fatmosphere 5 0.9 N 1 198.8 N 5 199.7 N
(1.13)
Note that the piston mass is almost irrelevant, and it is common when solving this sort of
problem to assume the mass of the piston is negligible.
Pressure is often quantified using a gauge, which generally compares the pressure being measured to the pressure of the surrounding atmosphere. For example, if
you measure the pressure of a car tire with a gauge and it reads 30.0 psi, this probably
means 30.0 psi above ambient pressure. The pressure inside the tire in absolute terms
is 44.7 psi (30.0 + 14.7) if the gauge is operating around sea level.
The distinction between “absolute pressure” and “gauge pressure” is important
enough that in English units there are commonly used symbols for each: psia stands
for “pounds per square inch absolute” and psig stands for “pounds per square inch
gauge.” The official recommendation from NIST is to place clarifiers on the quantity
“Gage” is sometimes
used as a secondary
spelling of the word
“gauge.”
NIST is the National
Institute of Standards
and Technology.
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22
Fundamentals of Chemical Engineering Thermodynamics
being measured (P) rather than on the units. Thus the NIST recommends the nomenclature Pg 5 30.0 psi and Pa 5 44.7 psi, rather than P 5 30.0 psig and P 5 44.7
psia. However, the psia and psig symbols remain in widespread use. Consequently
using the symbol “psi” without somehow specifying “absolute” or “gauge” can cause
confusion.
In this book, pressure is always expressed on an absolute basis, unless explicitly
stated otherwise.
1.4.3 Work
Work (W) can be quantified as the product of the distance moved (l) times the
magnitude of the force opposing the motion (F ). In many applications, the force
opposing the motion is not constant, so it’s necessary to express the definition in
differential form as
dW 5 F dl
Throughout this book,
a dot over a symbol
signifies a rate. Thus W
represents work, and
·
W represents work per
unit time.
examPle 1‑3
(1.14)
The SI unit for work (and more fundamentally, for energy) is the Joule, which is
defined as the work required to move an object one meter against an opposing force
of one Newton (1 J = 1 N · m).
In some applications, it is convenient to quantify the power, which is the rate at
which work is done:
·
W 5 Fv
(1.15)
·
In this equation, W stands for the power, and v is the velocity. You can make the
connection between Equations 1.14 and 1.15 clearer by considering that velocity is
equivalent to the derivative of position or dlydt. The SI unit for power is the Watt,
which is defined as 1 J/s.
The following example applies Equation 1.14 to a familiar everyday application.
LIFTINg A BOx
A mover lifts a 50 lbm box off the ground and places it on a truck (Figure 1-21). If the floor
of the truck is 4 feet off the ground, how much work was required to lift the box?
50 lbs
Thermo-Movers
4 ft
Figure 1‑21 A 50 lbm box is lifted off the ground and placed on the
flatbed of a truck.
SOLUTION:
Step 1 Define a system
Define the box as the system.
Step 2 Apply definition of work
The system is moving upward. The force opposing the motion is gravity, and is thus given by
F 5 Mg
(1.16)
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C H A P T E R 1 Introduction
Consequently, work is given by
dW 5 F dl 5 Mg dl
(1.17)
In this case, M and g are constant, and integration of Equation 1.17 is
#
1
2
ft
Mg dl 5 s50 lbmd 32.174 2 s4 ft 2 0 ftd
W5
s
l50 ft
l54 ft
1
1 lbf
lbm ? ft
32.174
s2
2
(1.18)
W 5 200 ft ? lbf
Note that this problem only examined the work required to lift the box up 4 feet; horizontal motion was not considered. Horizontal motion requires work when there is friction opposing the motion (e.g., to push the box along the flatbed of the truck one would
have to do work to overcome friction).
Example 1-3 reviews material that should be familiar to most readers from introductory physics. However, one point worth examining is the sign of l (length). It
probably seems natural that dl was positive here since the box was moving up. However, in many cases it might seem arbitrary which direction is positive and which
is negative. We wish to establish a sign convention where W > 0 is when energy is
transferred TO the system in the form of work and W < 0 is when energy is transferred FROM the system to the surroundings in the form of work. To accomplish
this, the distance moved (dl) is defined as positive when the motion is in the same
direction as the external force acting on the system and negative when the motion is
in the opposite direction of the external applied force.
Consider this definition as it applies to two possible systems (the box and the
mover):
W > 0 for work done
ON system
W < 0 for work done
BY system
Be aware that some
books and references
use the opposite sign
convention.
When the box was the system, there were two external forces—the force applied by
the mover, which was acting upward, and the force of gravity, acting downward. The
force applied by the mover was larger; otherwise the box wouldn’t have moved up.
The motion was upward, and the NET external force was also upward. Thus the displacement was POSITIVE 4 feet, as illustrated in Equation 1.18, reflecting that energy
was added to the system by the mover.
If we defined the mover as the system, then the force of gravity on the box would be
an external force acting on the system. In this case there is no other external applied
force; the mover is producing the upward force internally. The motion would be upward, but the external force would be acting downward. Consequently in this case we
would say the displacement is NEGATIVE 4 feet, and the work is 2200 ft · lbf, reflecting
the fact that the system (the mover) is doing work on the surroundings (the box).
Example 1-3 illustrates the basic concept of work but is not really a chemical
engineering application. There are three categories of work that are of great significance throughout this book.
1. Work of expansion/contraction
2. Flow work
3. Shaft work
These are examined through the following examples.
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23
24
Fundamentals of Chemical Engineering Thermodynamics
examPle 1‑4
ExPANSION DUE TO hEATINg
A gas is confined in a piston-cylinder device and has an initial volume of 1 liter. It is
heated and expands to a volume of 2 liters (Figure 1-22). Assuming the external pressure
from the atmosphere is P 5 1 bar and the piston has negligible mass and negligible friction, how much work was done by the gas?
Patm
Patm
V2 5 2 L
V1 5 1 L
Figure 1‑22 A gas expands from V 5 1 L to V 5 2 L
against a constant pressure.
SOLUTION:
Step 1 Define a system
At a quick glance, this problem might look hopeless: The known pressure (1 bar) represents force per unit area. Without knowing the area or diameter of the cylinder, how can
we determine the force opposing the motion?
However, if we approach the problem methodically, by defining the gas inside the
cylinder as the system and applying definitions, we will find the problem can be solved
without calculating force explicitly.
Step 2 Apply definitions of force and pressure
By the definition in Equation 1.14, work is
dW 5 F dl
PITFALL
PREVENTION
Keep in mind that a
physical dimension
of a system, like
length (l), area (A) or
volume (V), is always
positive, but a change
in a dimension (dl, dA,
or dV) can be positive
or negative.
Force is unknown, but pressure is known, so it makes sense to apply the relationship
between F and P: pressure (P) is defined as force (F ) divided by cross-sectional area (A).
Consequently,
dW 5 PA dl
(1.19)
Step 3 Relate length to volume
Area (A) times length (l) is volume—thus the term A dl in Equation 1.19 is equivalent to
the change in volume of the gas.
A common mistake at this point would be to overlook the importance of the sign of
dl. Here the system is the gas; thus the system is driving the motion upward, and the external force is the atmospheric pressure opposing the motion. Consequently, the change
in l is negative (the direction of the motion is the opposite of the direction of the external
force), but the change in volume (V) is positive, so dV = 2A dl. Introducing this observation into Equation 1.20 gives
dW 5 2P dV
(1.20)
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 1 Introduction
Step 4 Solve the differential equation
The pressure opposing the motion is the external pressure of the atmosphere and is a
constant 1 bar. Consequently, Equation 1.20 integrates to
W52
#
V52 L
P dV
V51 L
(1.21)
5 2s1 bards2 L 2 1 Ld
1
10 5 Pa
bar
21
N
m2
Pa
1
21
1 m3
1000 L
2 1N1?Jm2
25
FOOD FOR
ThOUghT
1-8
How would the math
in this example be
different if, instead
of expanding, the
gas were being
compressed?
W 5 2100 J
Reviewing Example 1-4, we see that in order to find the work, we needed to
know the pressure opposing the motion, but it was not necessary to know any dimensions of the cylinder other than its total volume. Indeed, the fact that the system
was specifically a cylinder full of gas was irrelevant; the mathematics would apply to
any system.
Consequently, when the total volume of a system changes, work is done and is quantified by
#
WEC 5 2 P dV
(1.22)
Expansion/contraction work is sometimes called “PV
work” in reference to
Equation 1.22. However we don’t recommend the term “PV
work,” since it could
plausibly refer either
to expansion/compression work or to flow
work.
where WEC stands for work of expansion or compression, V is the total volume of the
system, and P is the force opposing the motion.
The next example explores the concept of flow work.
PIPE FLOW
Pure water flows through a horizontal section of pipe that is 5 feet long and 1 inch in
diameter. The water entering the pipe has a flow rate of 100 kg/min with a temperature
of 100°C and pressure of 2 bar. The exiting water also has a flow rate of 100 kg/min and
temperature of 100°C, but the pressure is 1.95 bar (Figure 1-23). The density of water at
these conditions is 961.5 kg/m3. Determine the rate at which work is done, both at the
pipe entrance and at the pipe exit.
examPle 1‑5
Water is widely used
in chemical processes
as a solvent and as a
coolant, so water flowing through a pipe is
a simple, yet common
and practical system
to consider.
5 ft
Water
100 kg/min
Tin 5 1008C
Pin 5 2 bar
10
Water
100 kg/min
Tout 5 1008C
Pout 5 1.95 bar
Figure 1‑23 Water flowing through a pipe experiences a gradual pressure drop.
SOLUTION:
Step 1 Define a system
We first want to find the rate at which work is done at the pipe entrance. If we define the
fluid in the pipe as the system, then the work we’re trying to calculate is identical to the
external work done on the system.
In design settings,
“given” information is
often on a mass basis
(such as 100 kg/min in
this problem), because
process design specifications are typically
on a mass basis (e.g.,
“We need to make
50 million pounds per
year of product.”).
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26
Fundamentals of Chemical Engineering Thermodynamics
We are given no information about where
the water entering the
pipe is coming from
(a tank? a river?
another section of
pipe?) and the
analysis here is not
dependent upon
knowing that.
When we look at
large volumes of fluid
(like the entire pipe)
the pressure is not
uniform, but the pressure at any particular
point in a fluid is
exerted equally in all
directions.
The velocity (v) is the
derivative of length
(l), or v = dlydt. So,
what we said previously about a sign
convention for l can
also be regarded as
true for v.
The specific volume
in this example is
V̂ 5 1/ 5 1/961.5 5
0.00104 m3/kg.
Specific volume is
discussed further in
Section 2.2.3.
The steam tables,
located in Appendix A,
give specific volume
and other properties of liquid water
and steam at many
conditions.
FOOD FOR
ThOUghT
1-9
Is the conversion into
Joules necessary, or is
0.208 bar ? m3/min a
valid answer?
Step 2 Apply definitions
As specified by Equation 1.15, the rate at which work is done, or power, is given by
·
W 5 F
Step 3 Examine the system to evaluate F
Figure 1-23 shows a small volume of water that is about to enter the pipe section. In order
to enter, it must overcome the pressure exerted by the water at the pipe entrance, which
opposes the motion.
The problem statement indicates that pressure gradually decreases along the length
of the pipe. However, as the section of pipe under consideration gets shorter, the change
in pressure along the length becomes smaller. The volume of liquid shown in Figure 1-23
(immediately outside the pipe entrance) is infinitesimally small, so it’s valid to model the
pressure as constant in this “slice” of water.
Consequently, Equation 1.15 can be expressed as
·
W 5 PA
(1.23)
where A is the cross-sectional area of the pipe, v is the velocity of the fluid, and P is the
·
pressure of the water at the exact point that it enters the system. What about the sign of W?
There are two ways we can rationalize the fact that this sign is positive.
1. Physically, energy is being transferred to the system from the surroundings—the system is the fluid inside the pipe, and the fluid immediately outside the system is pushing the system.
2. The external force and the motion are both acting in the same direction—in
Figure 1-23, they both act from left to right. In the discussion following Example 1-3,
we established that l (length travelled) is positive if the direction of travel and the
direction of the applied force are the same.
Step 4 Relate unknown velocity to known mass flow rate
The linear velocity of the liquid times the cross-sectional area of the pipe is equal to the
·
volumetric flow rate (V) of the liquid.
·
·
W 5 PV
(1.24)
·
Here, volumetric flow rate is not given but mass flow rate (m) is. You’re likely accustomed to relating to mass through the density, (defined as mass per unit volume), which
was given in this example as 5 961.5 kg/m3. In this book, we will more commonly use
the specific volume sV̂ d, which is defined as volume per unit mass. Density and specific
volume are reciprocals of each other, and either can be used here.
·
m
·
·
· Vˆ d
5 P sm
(1.25)
W 5 PV 5 P
1 2
Step 5 Solve equation
Inserting the given value of density gives
1 2
kg
100
·
m
min
·
5 s2 bard
W5P
kg
961.5 3
m
1 2
1
bar ? m3
·
W 5 0.208
min
21
105 Pa
1 bar
21
5 0.208
N
m2
Pa
1
bar ? m3
min
(1.26)
J
1J
5 20,800
1
2
2 1 Nm
min
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 1 Introduction
Step 6 Calculate work at exit
In order to leave the pipe, the water must overcome the pressure exerted by whatever
is immediately downstream of the pipe. If we go through steps 1 through 5, we see
that the derivation of Equation 1.25 still applies to the pipe exit, but there are two
differences:
1. The pressure at the outlet is 1.95 bar, not 2 bar.
2. The sign of the work is now negative. The motion is from left to right in Figure 1-23,
but the external applied force is acting from right to left. Because the directions
are opposite, the sign of v, and consequently of W, is negative. Another rationale is
W < 0, because the system (the fluid in the pipe) is doing work on to the surroundings
(the fluid immediately outside the pipe).
27
Chapter 3 introduces
an energy balance
equation that we will
use extensively. In this
equation, flow work
is accounted for using
the enthalpy (H),
which is formally
defined in Section 2.3.1.
Consequently the work done at the pipe outlet is:
1 2
kg
100
·
m
min
·
·
W 5 PV 5 P
5 2s1.95 bard
kg
961.5 3
m
1 2
1
bar ? m3
·
W 5 0.203
min
21
105 Pa
1 bar
21
N
m2
Pa
1
21
5 20.203
bar ? m3
min
(1.27)
2
1J
J
5 220,300
1N ? m
min
An important observation about Example 1-5 is that the expression for flow work
turned out to be independent of pipe area, or any other aspect of the geometry of the
system. Consequently, Equation 1.28 is applicable to any material stream.
Flow work is given by
·
·
· sPVˆ d
W 5 PV 5 m
(1.28)
·
·
where W is the rate at which work is done, P is pressure, V is volumetric flow rate, m· is
mass flow rate, and V̂ is specific volume. Pressure and flow rate are evaluated for the
fluid at the exact point that it enters or leaves the system.
Pressure times volume is a quantity that
occurs frequently in
thermodynamics. Volume can be expressed
in absolute terms
(e.g., m3), a molar
basis (m3/mol) or a
specific basis (m3/kg),
as discussed further in
Section 2.2.3.
Throughout this book,
the hat signifies a
property expressed
on a mass basis (e.g.,
specific volume) and
the underline signifies
a property expressed
on a molar basis (e.g.,
molar volume V).
ShaFT WorK
Shaft work is characterized by rotary, rather than linear, motion. Work is as always
equal to the distance travelled times the force opposing the motion, but “distance”
is measured by revolutions, as indicated in Equation 1.3. Many chemical processes
involve shaft work being either added to, or produced by, a moving fluid; the pump
and turbine in the Rankine cycle are prime examples. The moving parts in pumps
and compressors are powered by shaft work. Conversely, the kinetic energy in moving liquids or gases can be converted into shaft work by devices like windmills and
turbines. Shaft work can immediately be recognized as equal to zero in any system
that has no moving parts.
Equations 1.22 and 1.28 relate expansion work and flow work to pressure and
volume. There is an analogous equation for shaft work but its derivation is more
complex and requires the concept of energy balances. This derivation is shown in
Example 3-8.
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28
Fundamentals of Chemical Engineering Thermodynamics
1.4.4 Kinetic Energy
Kinetic energy (K.E.) is stored energy that takes the form of motion. It is quantified by
K.E. 5
1
Mv 2
2
(1.29)
In this expression, M is the mass of the object, and v is the velocity of the object.
The connection between kinetic energy and the potential to do work is straightforward; if an object is in motion, and there is a force opposing the motion, then
work is being done. For example, consider a ball at the base of a hill (see Figure 1-24).
It takes work to move the ball up the hill against the opposing force of gravity, and a
person could supply this work by pushing the ball with his or her hand or foot. But
if the ball, instead of being stationary at the bottom of the hill, is rolling along a flat
surface toward the hill, then the ball rolls at least partway up the hill without any external work required; the kinetic energy of the ball is equivalent to the work done by
a person’s hand. Kinetic energy also can be transferred to other objects in the form
of work, as when the moving billiard ball strikes the stationary one in Figure 1-9.
(a)
(b)
Figure 1‑24 (a) A ball is moved up a hill by adding work. (b) A
ball’s stored energy, in the form of kinetic energy, allows it to climb
the hill without addition of external work.
Taking the example a step further, we can use Equation 1.14 to determine how
much work was done when the ball rolled up the hill, but where did the work “go?”
The first law of thermodynamics states that energy cannot be created or destroyed,
and (neglecting friction) the ball did not transfer the energy to any other object, so
the energy must be stored inside the object in some form. As the ball rolls up the hill,
it gains potential energy, which is discussed in the next section.
In theory, Equation 1.29 is applicable to a particle of any size, but in this book, we
draw a distinction between macroscopic and microscopic kinetic energy. Consider a
glass of water. We know that the individual water molecules are constantly in motion. Indeed individual water molecules can move with extremely high velocity, and
they are constantly colliding, accelerating, decelerating, and changing directions. In
addition to translational motion (motion of the entire molecule through space), molecules with more than one atom also exhibit vibrational motion (e.g., bonds stretching and compressing) and rotational motion (e.g., the free rotation of single bonds.)
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C H A P T E R 1 Introduction
However, all of this molecular motion does not contribute to macroscopic motion;
the water (as a whole) appears stationary. If we pour out the water, the water as it is
moving has a measurable macroscopic velocity, and the macroscopic kinetic energy
of the water can be calculated using Equation 1.29.
If we knew the velocity of a molecule, we could compute its kinetic energy using
Equation 1.29. However, practically speaking, it’s unrealistic to quantify the kinetic energy of each individual molecule in a glass. Instead, all of the energy stored as kinetic
energy of individual molecules is lumped into a macroscopic property called internal
energy, described in Section 1.4.6. Thus, even though molecular motion does, strictly
speaking, represent kinetic energy, throughout the rest of this book we will use the term
“kinetic energy” to describe only the macroscopic motion of an entire object or system.
29
An 8-ounce glass of
water contains
approximately 8 3 10 24
individual water
molecules.
1.4.5 Potential Energy
Potential energy is stored energy associated with an object’s position in a force field.
For example, consider two objects that are initially both stationary: They have no
kinetic energy, so if they start moving toward each other with no outside intervention, it would appear that kinetic energy is being spontaneously created. However,
the motion is not arbitrary; the objects move toward each other because they are
attracted to each other by gravitational or electromagnetic forces.
Consequently, objects that are located within significant electromagnetic or
gravitational force fields are said to have potential energy; the attractive forces acting upon them have the potential to add kinetic energy to the objects, and in turn, to
do work. The waterwheel is a practical example of a system that converts potential
energy into work. Water at the top of a slope has potential energy. If the water was
allowed to flow downhill in a natural channel the potential energy might be converted into kinetic energy—the water would flow faster. Instead the stream is
re-routed to the top of the waterwheel. This water is captured in buckets, and as
the force of gravity lowers the bucket, the axle turns. Historically, waterwheels were
largely used to grind grain into flour, but conceptually, the shaft work produced by a
waterwheel can be used to power any device that requires rotary motion.
Like kinetic energy, potential energy exists on both a macroscopic and microscopic
level. Intermolecular attractions and repulsions give rise to potential energy on a microscopic level, but for our purposes the microscopic potential and kinetic energy of individual molecules are lumped together into internal energy, described in the next section.
Potential energy resulting from electromagnetic fields, while of great importance in physics and some engineering disciplines, is not typically a significant consideration in chemical engineering thermodynamics. In this book we will primarily
be concerned with macroscopic potential energy resulting from an object’s position
in Earth’s gravitation force field..
FOOD FOR
ThOUghT
1-10
The river at the top of
Niagara Falls appears
to flow much faster
than the river leading
away from the falls.
How can the water
at the top of the falls
have BOTH more
potential energy and
more kinetic energy
than the water at the
bottom?
Potential energy (P.E.) resulting from gravity is quantified by
P.E. 5 M gh
(1.30)
where M is the mass of the object, g the acceleration due to gravity, and h the height.
In using Equation 1.30, “height” is usually regarded as equal to 0 at the ground,
as in Example 1-6 below. Conceptually, however, height is measured on a relative
scale, and any convenient point (e.g., the top of a table or the roof of a building) can
be defined as h = 0.
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30
Fundamentals of Chemical Engineering Thermodynamics
examPle 1‑6
OBJEC T IN FREE FALL
An object is dropped from a height of 50 feet off the ground. Initially it has no velocity
(Figure 1-25). Assuming no forces act on the object other than gravity, what is its velocity
when it hits the ground?
vinitial 5 0
50 ft
vfinal 5 ?
Figure 1‑25 An object experiences
free fall from a height of 50 feet.
SOLUTION:
Step 1 Define a system
The object is the system. The process ends at the moment the object strikes the ground.
Step 2 Apply the concept of conservation of energy
The first law of thermodynamics states that energy can be converted from one form into
another, but it cannot be created or destroyed. Initially, the object has zero kinetic energy
(vinit = 0) but has potential energy as given by Equation 1.30. At the end of the process
the object has zero potential energy (hfinal = 0) and kinetic energy as given by Equation
1.29. Since there are no other forces acting upon the object, we can assume the initial
potential energy equals the kinetic energy at the end of the process
Mghinit 5
1
M 2final
2
(1.31)
Step 3 Solve the equation
The mass of the object cancels. Inserting known values into the left-hand side gives
132.174 s 2s50 ftd 5 2 v
ft
1
2
2
final
(1.32)
This is a workable equation in that the units of the left-hand side are ft2/s2, so when
we multiply through by 2 and take the square root of both sides, we will be left with a
velocity in ft/s.
final 5 56.7
ft
s
(1.33)
You’ve likely solved problems like this one in introductory physics, using a different
approach that involves applying the facts that acceleration is the first derivative of velocity and velocity is the first derivative of position. This approach is shown in an expanded
version of the problem located in the electronic supplements. It’s instructive to see that
both approaches produce equivalent answers.
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C H A P T E R 1 Introduction
1.4.6 Internal Energy
The internal energy of a substance consists of the energy stored by individual molecules. As mentioned in Sections 1.4.4 and 1.4.5, materials store large quantities of
energy in the form of microscopic potential and kinetic energy; these are the components of internal energy. The “potential to do work” that accompanies internal
energy is more subtle than it is for macroscopic potential and kinetic energy, but it
does exist. The Rankine cycle described in Section 1.2.3 is essentially a machine used
to convert the internal energy of a fuel (e.g., coal or oil) into shaft work. Conversely,
shaft work can be converted into internal energy: pumps and compressors are unit
operations that are used to increase the pressure of a fluid; this requires the addition
of work, and the work added leads directly to an increase in the internal energy of
the fluid.
In principle, one might envision measuring internal energy on an absolute
scale:
If the position and velocity of all molecules were known, internal energy can be
quantified using Equations 1.29 and 1.30.
A material in which there was no molecular motion, and no intermolecular
attractions and repulsions between particles, would have zero internal energy.
There would be no such thing as negative internal energy.
Practically speaking, one cannot imagine implementing the above strategy,
due to the number of molecules and the complexity of knowing their exact velocity and position at any given time. Consequently, though internal energy is
understood at the molecular level, it is normally quantified with macroscopic
measurement.
We define the property specific internal energy sÛd as energy per unit mass
(e.g., kJ/kg).
A “reference state” is chosen for which Û = 0.
Internal energy at other conditions is determined by measuring the energy
added or removed for a particular change in state (e.g., how much energy did it
take to raise the temperature 5 degrees above the temperature of the reference
state, 10 degrees above, etc.).
Because internal energy is measured relative to a reference state, negative values of Û are possible, even though in an absolute sense all materials have positive
amounts of stored energy. The specific choice of reference state is not fundamentally
important; what is crucial is that a single reference state is used within each calculation. Reference states are explored in the following example.
INTERNAL ENERgy OF LIqUID WATER AND STEAM
examPle 1‑7
Your team is designing a process that has six streams containing liquid water or steam at
different temperatures and pressures, and you need to know the specific internal energy
for each. Your teammate has given you the information in Table 1-1, but you need to fill in
the missing data. Using the steam tables in Appendix A, determine the values that should
go in the two empty cells.
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31
32
Fundamentals of Chemical Engineering Thermodynamics
Table 1‑1 Specific internal energy of liquid water and steam at various conditions.
PITFALL
PREVENTION
We can’t use data
from the steam tables
directly. They have a
different reference
state than the data
given in this example.
The “triple point” of a
compound is discussed
in Section 2.2.1.
Physically, Û4 2 Û5
represents the amount
of internal energy
that must be added to
a kilogram of water
in order to convert it
from liquid at
T = 100°C and P = 5
bar to vapor at T =
100°C and P = 1 bar.
In step 2, the calculations use the same
reference state as the
steam tables. In step 3,
the calculations use
the same reference
state as Table 1-1.
Stream
Temperature (°C)
Pressure (bar)
Phase
Û (kJ/kg)
1
25
1.0
Liquid
0 (reference state)
2
50
1.0
Liquid
104.5
3
75
1.0
Liquid
209.2
4
100
1.0
Vapor
2401.4
5
100
5.0
Liquid
6
200
5.0
Vapor
SOLUTION:
Step 1 Collect data
According to Appendix A-4, for stream 5 (liquid water at P 5 5 bar and T 5 100°C) the
specific internal energy is Û5 5 418.9 kJ/kg, and according to Appendix A-3, for stream 6
(steam at P 5 5 bar and T 5 200°C) the specific internal energy is Û6 5 2643.3 kJ/kg. The
conditions for the liquid water in streams 1, 2, and 3 are not shown in the steam tables.
However, stream 4 (steam at P 5 1 bar and T 5 100°C) is shown, and its value is Û4 5
2506.2 kJ/kg. This is different from the value in your teammate’s table.
Step 2 Compute changes in specific internal energy
The steam tables use liquid water at the triple point (T 5 0.01°C, P 5 611.7 Pa) as a
reference state, while your teammate’s table uses liquid water at approximately ambient
conditions (T 5 25°C and P 5 1 bar) as a reference state. The data we fill into the two
empty cells must use the same reference state as the rest of the table, so that the team can
use this table for accurate, self-consistent design calculations. How can we convert the
steam table data to your teammate’s alternative reference state?
Engineers are primarily concerned with changes in the conditions of materials—how
much energy does it take to heat water from T 5 100°C to T 5 200°C, or compress it
from P 5 1 bar to P 5 5 bar? The answers to questions like these do not depend upon
what we choose as a reference state. Thus, while the values of Û4 and Û5 depend upon
the reference state, the difference between them does not. Using the data from the steam
tables,
Û4 2 Û5 5 2506.2 2 418.9
kJ
kJ
5 2087.3
kg
kg
(1.34)
Û4 2 Û6 5 2506.2 2 2643.3
kJ
kJ
5 2137.1
kg
kg
(1.35)
and
Step 3 Determine Û5 and Û6 in the team’s reference state
According to Table 1-1, the specific internal energy of stream 4 is Û4 5 2401.4 kJ/kg. The
value of Û4 2 Û5 was calculated using a different reference state, but is the same regardless of the reference state. We can combine these to find the value of Û5 for the reference
state used in Table 1-1.
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C H A P T E R 1 Introduction
Û4 2 Û5 5 2087.3
2401.4
kJ
kg
kJ
kJ
2 Û5 5 2087.3
kg
kg
Û5 5 314.1
33
(1.36)
kJ
kg
Applying a similar procedure to stream 6:
Û4 2 Û6 5 2137.1
2401.4
kJ
kg
kJ
kJ
2 Û6 5 2137.1
kg
kg
Û6 5 2538.5
(1.37)
kJ
kg
The steam tables, which were used in Example 1-7, give data for Û and other
properties of water, in the liquid and vapor phases at a broad range of conditions.
Chapter 6 discusses how changes in internal energy can be estimated in circumstances where experimental data does not exist.
The tables in
Appendix A are
typically called “steam
tables” even though
they contain properties
of liquid water as well
as steam.
1.4.7 heat
(a)
Like W, we define Q
to be positive when
energy enters the
system and negative
when energy leaves
the system.
Gina Tierno
Gina Tierno
Heat is a form of energy transfer that is distinct from mechanical work in that the
energy transfer occurs without accompanying macroscopic motion. Motion, and
consequently work, occur because of an imbalance in force. Heat transfer occurs
because of an imbalance in temperature. We use the symbol Q to represent heat
energy entering the system.
The phenomenon of heat transfer can be understood, on a molecular level, as an
extension of the previous discussion on internal energy. Recall that internal energy is
a macroscopic property that encompasses microscopic potential and kinetic energy.
The molecules in liquid water move more quickly than the molecules in an ice cube.
If ice cubes are placed in water (Figure 1-26a), collisions between slow-moving (ice)
molecules and fast-moving (liquid water) molecules result. Since one gram of water
(b)
Figure 1‑26 (a) A glass of water immediately after ice cubes are dropped in and (b) the
glass of water after it reaches equilibrium. Temperatures are displayed in degrees Celsius.
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34
Fundamentals of Chemical Engineering Thermodynamics
contains .3 3 1022 molecules, the number of individual collisions is almost unimaginably large. The net effect of all these collisions is to “even out” the internal energy:
Macroscopically, the ice cube melts and the liquid water loses heat and decreases
temperature.
On a molecular level (Figure 1-27), the ice molecules gain energy and the liquid
water molecules lose energy.
Solid ice
Liquid water
Liquid water
Solid ice
Figure 1‑27 Molecular-scale imagining of the melting of an ice cube in
liquid water.
FOOD FOR
ThOUghT
1-11
According to the
thermocouple, the
temperature of the
ice-water equilibrium
mixture in Figure
1-26b is 0.5 degrees
Celsius. Shouldn’t it
be exactly 0?
If a single ice cube were dropped into a glass of hot water, the ice cube would
melt completely. At this point, the molecules that were originally part of the ice and
those that were originally part of the water are indistinguishable. The molecules are
now all liquid water and their internal energy, on average, falls somewhere between
the hot water’s original internal energy and the ice’s original internal energy. Their
temperature, too, falls between the original temperatures of the ice and liquid water.
In Figure 1-26a, there is a large amount of ice and the liquid water is cool (16.6°C)
to begin with. Here again, internal energy is transferred (through molecular collisions) from the liquid water to the ice, melting the ice and cooling the liquid water.
But in this experiment, the liquid water reached its freezing point before all the ice
had melted, producing an ice-water mixture in which the ice and water are both at
the freezing point (measured as 0.5°C in Figure 1-26b). This final state provides an
example of thermal equilibrium; equilibrium was introduced in Section 1.3.2.
This example illustrates a familiar everyday phenomenon; that when highenergy “hot” objects and low-energy “cold” objects are brought into contact, energy
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C H A P T E R 1 Introduction
flows from the “hot” object to the “cold” one. Temperature is the property that is
used to quantify how “hot” something is, allowing us to predict whether heat transfer
will occur and quantify the physical effects of the heat transfer. While temperature
is a familiar quantity in an everyday sense, formal definitions for temperature are
presented in Section 1.4.8.
Throughout this section we have observed that the various forms of stored
energy (kinetic, potential and internal) are recognizable as energy because they can
be converted into work. Heat, in turn, is recognizable as energy because it is a transfer of stored energy from one location to another.
While heat transfer is a process distinct from work, the two routinely occur
simultaneously. For example, heating something generally causes it to expand, and
expansion implies work unless there is no pressure opposing the expansion.
1.4.8 Temperature
Internal energy was described in Section 1.4.6 as microscopic kinetic and potential
energy; the energy of individual molecules. A difficulty is that microscopic potential
and kinetic energy is not something that can be measured directly. The virtue of
temperature is that it is a readily measurable property that allows us to benchmark
the internal energy of a system.
The Celsius temperature scale was originally defined as
The freezing point of water at atmospheric pressure is 0°C.
The boiling point of water at atmospheric pressure is 100°C.
The phenomenon of heat transfer is what makes a temperature scale meaningful. Two objects are said to be in “thermal equilibrium” if no heat is transferred
between them, even though they are in contact such that heat transfer could occur.
By definition, materials in thermal equilibrium are said to be at the same temperature. Thermal equilibrium has been observed to be transitive; that is, if material A is in thermal equilibrium with material B, then any other material that is
in thermal equilibrium with material A will also be in thermal equilibrium with
material B.
Thus, any material that is in thermal equilibrium with water at its normal freezing point is said to have a temperature of 0°C. Any material that would transfer heat
TO water at its normal freezing point has a temperature higher than 0°C, and any
material to which heat would transfer FROM water at its normal freezing point has
a temperature lower than 0°C. The definition of two points permits construction of
a linear scale for temperature in between these two points. The transitive nature of
thermal equilibrium allows us to calibrate instruments that measure temperature
(e.g., we know that any two objects that provide the same reading on a thermometer
will be in thermal equilibrium with each other).
While temperature is related to internal energy, it is not a simple or linear relationship. For example, liquid water at its freezing point and ice at its melting point are
in thermal equilibrium with each other, and therefore at the same temperature, but
the liquid water has far more internal energy per unit mass than does the ice. Temperature is best understood not as a “measurement of internal energy” but as a measurement of the propensity of a material to transfer heat to or from other materials.
The Celsius temperature scale was first devised before the concept of an absolute
temperature scale existed. We can imagine a state, termed absolute zero, in which
there is zero molecular motion and zero stored energy. A material that has zero
stored energy has no ability to transfer heat to any other material, and therefore has,
in an absolute sense, zero temperature.
35
The Celsius
temperature scale was
originally called
“Centrigrade,”
because it had 100
“gradations” between
the freezing point and
boiling point of water.
In 1948, at the Ninth
International Conference on Weights
and Measures, it was
formally renamed
“Celsius,” in honor
of Anders Celsius.
(Olson, 1998)
This definition of
Celsius has formally
been replaced, though
the new definition was
crafted to be practically consistent with
this one.
The freezing point and
boiling point of a pure
substance measured
at atmospheric pressure are also termed
the “normal freezing
point” and “normal
boiling point.”
In Section 2.3.2, the
relationship between
internal energy and
temperature is
discussed further.
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Fundamentals of Chemical Engineering Thermodynamics
The Kelvin temperature scale is named
after Sir William
Thompson, whose title
was Baron Kelvin of
Largs, and is consequently often referred
to as “Lord Kelvin.”
(Olson, 1998)
The “triple point” of a
compound is discussed
in Section 2.2.1.
The Kelvin temperature scale is defined as
Absolute zero corresponds to 0 K.
The temperature at the triple point of water is defined as 273.16 K.
“One Kelvin” is defined as 1/273.16 of the temperature difference between these
points. Temperature in degrees Celsius is now formally defined as temperature in
Kelvin plus 273.15. This definition aligns almost perfectly with the original “centigrade” definition. By the original scale, the normal boiling point of water was exactly
100°C by definition. When measured on the temperature scale as currently defined,
the normal boiling point of water has been reported at various values ranging from
373.15 to 373.20 K, which corresponds to 100.00 to 100.05°C (Arce, 1998; Fandary,
1999; Rajendran, 1989). For common use, it remains accurate to say that at atmospheric pressure, water freezes at 0°C and boils at 100°C.
The Rankine scale, like the Kelvin, measures temperature on an absolute basis;
0°R represents absolute zero. Figure 1-28 illustrates the relationships between the
Kelvin, Celsius, Fahrenheit, and Rankine temperature scales, and the following
equations summarize the conversions between them.
TCelsius 5 TKelvin 2 273.15
TRankine 5
(1.38)
9
T
5 Kelvin
(1.39)
TFahrenheit 5 TRankine 2 459.67
Rankine
Melting point of
copper (Perry, 1997)
Fahrenheit
(1.40)
Celsius
Kelvin
24408R
19808F
10838C
1360 K
809.78R
3508F
176.78C
449.8 K
671.78R
558.38R
491.78R
2128F
98.68F
328F
1008C
378C
08C
373.2 K
310.2 K
273.2 K
162.48R
2297.38F
21838C
90.2 K
08R
2459.78F
2273.28F
0K
A common baking
temperature
Normal boiling
point of water
Human body
temperature
Normal freezing
point of water
Normal boiling point
of oxygen (Perry, 1997)
Absolute zero
Temperature Scales
Based on FSU website
36
Figure 1‑28 Comparison of Fahrenheit, Celsius, Kelvin, and Rankine temperature scales.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 1 Introduction
The concept of an absolute temperature has broad utility in chemical engineering thermodynamics. The ideal gas law, discussed in Section 2.3.3, relates temperature, pressure and molar volume to each other, but requires that all three quantities
(T, P, and V) are measured on an absolute scale. Many additional thermodynamics
properties (e.g., entropy, Gibbs energy) developed later in this book also depend
upon an absolute temperature. A simple point, but one that often causes confusion,
is addressed in the following example.
A ChANgE IN TEMPERATURE
examPle 1‑8
On a stove, water is heated from room temperature T 5 25°C to boiling temperature
T 5 100°C. Determine ΔT in degrees Celsius and in degrees Kelvin.
SOLUTION:
In Celsius:
DT 5 Tfinal 2 Tinitial 5 100 2 258C 5 758C
(1.41)
In Kelvin:
DT 5 Tfinal 2 Tinitial 5 s100 1 273.15 Kd 2 s25 1 273.15 Kd
(1.42)
5 75 K
As mentioned above, in many thermodynamics applications, temperature (T)
must be expressed on an absolute scale. A danger is becoming so accustomed to
adding 273.15 to any Celsius temperature that one carelessly does so for a change in
temperature. For example “the water temperature increased by 75°C,” does not convert to “the water temperature increased by 348.15 K.” Example 1-8 illustrates that a
change in temperature (ΔT) is identical on the Celsius and Kelvin scales, because the
conversion factor cancels. A similar demonstration could be made for the Fahrenheit
and Rankine scales.
1.4.9 Overview of the Forms of Energy
Conceptually, the crucial points in this section include:
Work occurs because of an imbalance in force, and is recognizable when motion
occurs against an opposing force.
Other forms of energy are recognizable and quantifiable through their potential
to be converted to and from work.
Kinetic energy and potential energy are the forms in which energy can be stored.
Internal energy is stored energy in the form of microscopic kinetic and potential
energy.
Heat is energy transferred due to an imbalance in temperature.
Quantitatively, the most important mathematical relationships in this section are
dW 5 F dl
(1.43)
dWEC 5 2P dV
(1.44)
1
K.E. 5 M 2
2
(1.45)
P.E. 5 Mgh
(1.46)
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37
38
Fundamentals of Chemical Engineering Thermodynamics
1.5 S U M M A R y O F C h A P T E R O N E
The first law of thermodynamics states that energy cannot be created or
destroyed, but it can be converted from one form into another.
Engineers design products and processes that accomplish specific tasks or meet
specific needs. Converting and using energy as efficiently as possible is a fundamental concern in design.
Energy can be stored in the forms of internal energy, kinetic energy and potential energy.
Energy can be transferred in the forms of heat (Q) and work (W ).
Work is the energy required to move something through a distance against an
opposing force.
Work occurs when there is an imbalance in force, and heat transfer occurs when
there is an imbalance in temperature.
For our purposes, three forms of work are significant: flow work, shaft work, and
expansion/contraction work.
Solving problems is facilitated by defining a system. A system must have clear,
unambiguous boundaries, such that we can recognize when mass or energy
enters or leaves the system.
W and Q are considered positive when energy is added to the system and negative when energy is removed from the system.
Solving problems is often facilitated by defining a process, which has a clear
beginning and a clear end.
The initial state is a description of the system at the beginning of the process,
and the final state is a description of the system at the end of the process.
A closed system is one in which no matter enters or leaves the system, and an
open system is one in which matter can cross the boundaries of the system.
In an isolated system, neither matter nor energy crosses the boundaries of the
system.
An adiabatic process is one in which no heat is added or removed.
An isothermal process is one in which temperature is constant with respect
to time, and an isothermal system is one in which the temperature is uniform
throughout the system.
An isobaric process is one in which pressure is constant with respect to time,
and an isobaric system is one in which the pressure is uniform throughout the
system.
An isochoric system has a constant volume.
A system is at steady state if ALL properties of the system remain constant with
respect to time.
A system is at equilibrium when there is no driving force present that will cause
the properties of the system to change.
The driving forces for change that we are concerned with in chemical engineering thermodynamics are differences in temperature, pressure, and fugacity.
It is possible for a system to be at steady state without being at equilibrium.
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C H A P T E R 1 Introduction
39
1.6 E x E R C I S E S
1-1.
1-2.
1-3.
Several systems are described below. For each system, go through each of the following adjectives, and
determine which ones apply: open, closed, adiabatic,
isolated, isochoric, isothermal, or isobaric.
A. An ice cube that has just been dropped into a
glass of warm water.
B. An aluminum can full of soda that has been fully
submerged in ice water for some time.
C. The inside of a closed, perfectly insulated cooler
that contains ice, water, and cans of soda.
D. A closed, reasonably well insulated cooler that
contains ice, water, and cans of soda.
E. Your body, over the last 24 hours.
F. Your body, over the last five years.
G. The Atlantic Ocean, over the last 24 hours.
H. The Atlantic Ocean, over the last million years.
Suppose a car, and everything inside it, is defined as
the system. A car is not necessarily an open or closed
system; it could be either, depending upon the specific process and circumstances under consideration.
A. While a car is being driven with the windows
rolled up, is the system open or closed? Which,
if any, of these apply: isolated, isochoric, isothermal, isobaric, adiabatic?
B. While a car is sitting in a garage, with no passengers inside, all windows rolled up and the engine
not running, is the system open or closed? Which,
if any, of these apply: isolated, isochoric, isothermal, isobaric, adiabatic?
C. Describe a process (different from either A or B)
in which the car can be considered a closed system.
D. Describe a process (different from either A or B)
in which the car can be considered an open system.
Several systems and processes are described below.
For each of them, determine whether the system is
at steady state, at equilibrium, or neither. Determine
also whether any of these adjectives are applicable
to the system: isothermal, isobaric, isochoric, adiabatic, closed, or open.
A
B
System
Process
An ice cube
The ice cube is inside a freezer,
where is has already been for
some time before the process
begins. The process lasts 24
hours, during which the freezer
door is never opened.
An ice cube
The ice cube is inside a freezer.
The freezer door is opened,
allowing room temperature air
in. The door is open for one
minute, and the process ends 10
minutes after the door is closed.
C
The air
inside a hot
air balloon
The balloon rises from the
ground to a height of
1000 feet.
D
The air
inside a hot
air balloon
The balloon floats, stationary, 1000 feet above the
ground. The air in the balloon
is maintained at a constant
temperature.
E
A wood
furnace and
its contents
Wood is piled inside the
furnace. When the process
begins, the wood is already
burning and the furnace and
its contents are at a uniform
T = 300°F. At the end of the
process, 10 minutes later, the
temperature is still a uniform
T = 300°F.
F
A section of
a water pipe
Water is flowing through
the pipe at a constant flow
rate and a constant, uniform
temperature.
G
The water in
a sink
The process begins immediately after the faucet is turned
off. The water is initially at
90°F, the ambient air is at 70°F.
The process continues for 24
hours, during which evaporation is negligible and no water
is added or removed.
H
Your body
You are stationary and
immersed in bathwater at
98.6°F throughout the process.
1-4.
Ray Bradbury’s novel title Fahrenheit 451 is based
upon the temperature at which paper, or more specifically books, burn. Express 451.0°F in Celsius,
Kelvin, and Rankine.
1-5.
Convert P = 5.00 atm into Pa, bar, and psia.
1-6.
Convert 10,000 ft-lbf of energy into BTU, Joules, and
kilojoules.
1-7.
Convert 5 megawatts of power into BTU/hr, ft-lbs/s,
and kJ/hr.
1-8.
Use the data in the steam tables (Appendix A) to
answer the following:
A. Find the change in internal energy when 50 kg
of steam at constant pressure P = 5 bar has its
temperature reduced from 500°C to 300°C.
B. Find the change in internal energy when 200
kg of liquid water at constant pressure P = 100
bar has its temperature increased from 240°C
to 300°C.
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40
Fundamentals of Chemical Engineering Thermodynamics
1-9.
Use the data in the steam tables to answer the
following:
A. Find the change in volume when 50 kg of steam,
initially at T = 200°C and P = 2 bar, is heated
and compressed into steam at T = 350°C and
P = 5 bar.
B. Find the change in volume when 100 kg of liquid
water at P = 100 bar and T = 300°C is converted
into 100 kg of steam at P = 5 bar and T = 300°C.
1-10. An object has a mass of 10 lbm.
A. Find the change in potential energy when it is lifted
from the ground to 25 feet above the ground.
B. Find the change in kinetic energy when it its
velocity decreases from 50 mph to 15 mph.
1-11. An object has a mass of 50 kg.
A. Find the change in potential energy when it is initially 10 meters above the ground and is raised to
25 meters above the ground.
B. Find the change in kinetic energy when it is initially stationary and is accelerated to 10 m/s.
1-12. Find the expansion work resulting when the volume
of a gas increases from 1 ft3 to 5 ft3 against a constant
pressure of 1 atm. Express the answer in Joules.
1-13. Find the expansion work done when the volume
of a gas is increased from 0.2 m3 to 1.5 m3 against
a constant pressure of 50 kPa. Express the answer
in ft · lbf.
1-14. Find the flow work added to a system by a fluid
that enters with a volumetric flow rate of 500 in3/s
at a pressure of 25 psia. Express the answer in
ft · lbf /min.
1-15. Find the flow work added to a system by a fluid
that enters with a volumetric flow rate of 500 L/min
at a pressure of 3 bar. Express the answer in Watts.
1.7 P R O B L E M S
1-16. The value g 5 9.81 m/s2 is specific to the force of
gravity on the surface of the Earth. The universal
formula for the force of gravitational attraction is
F5G
m1 m2
r2
where m1 and m2 are the masses of the two objects,
r is the distance between the centers of the two objects, and G is the universal gravitation constant,
G = 6.674 3 10211 N(m/kg)2.
A. Research the diameters and masses of the Earth
and Jupiter.
B. Demonstrate that F = m(9.81 m/s2) is a valid
relationship on the surface of the Earth.
C. Determine the force of gravity acting on a 1000 kg
satellite that is 2000 miles above the surface of
the Earth.
D. One of the authors of this book has a mass of
200 lbm. If he was on the surface of Jupiter,
what gravitational force in lbf would be acting
on him?
1-17. A gas at T = 300 K and P = 1 bar is contained in
a rigid, rectangular vessel that is 2 meters long,
1 meter wide, and 1 meter deep. How much force
does the gas exert on the walls of the container?
1-18. A car weighs 3000 lbm and is travelling 60 mph
when it has to make an emergency stop. The car
comes to a stop 5 seconds after the brakes are
applied.
A. Assuming the rate of deceleration is constant,
what force is required?
B. Assuming the rate of deceleration is constant,
how much distance is covered before the car
comes to a stop?
1-19. Solar panels are installed on a rectangular flat roof.
The roof is 15 feet by 30 feet, and the mass of the
panels and framing is 900 lbm.
A. Assuming the weight of the panels is evenly distributed over the roof, how much pressure does
the solar panel array place on the roof?
B. The density of fallen snow varies; here assume
its ~30% of the density of liquid water. Estimate
the total pressure on the roof if 4 inches of snow
fall on top of the solar panels.
1-20. A box has a mass of 20 kg, and a building has a
height of 15 meters.
A. Find the force of gravity acting on the box.
B. Find the work required to lift the box from the
ground to the roof of the building.
C. Find the potential energy of the box when it is on
the roof of the building.
D. If the box is dropped off the roof of the building,
find the kinetic energy and velocity of the box
when it hits the ground.
1-21. 100 kg of steam is enclosed in a piston-cylinder
device, initially at 300°C and 5 bar. It expands and
cools to 200°C and 1 bar.
A. What is the change in internal energy of the
steam in this process?
B. If the external pressure is constant at 1 bar,
how much work was done by the steam on the
surroundings?
C. Research and briefly describe at least two examples of machines, either historical or currently in
use, that harness the energy in steam and convert
it into work. Any form of work is acceptable; you
needn’t confine your research to expansion work
(which was examined in parts A and B).
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C H A P T E R 1 Introduction
1-22. A. An object is dropped from a height of 20 feet off
the ground. What is its velocity when it hits the
ground?
B. Instead of being dropped, the object is thrown
down, such that when it is 20 feet off the ground,
it already has an initial velocity of 20 ft/s straight
down. What is its velocity when it hits the
ground?
C. What did you assume in answering parts A and
B? Give at least three examples of objects for
which your assumptions are very good, and at
least one example of an object for which your
assumptions would fail badly.
1-23. An airplane is 20,000 feet above the ground when
a 100 kg object is dropped from it. If there were no
such thing as air resistance, what would the vertical
velocity and kinetic energy of the dropped object be
when it hits the ground?
1-24. A filtration system continuously removes water
from a swimming pool, passes the water through
filters, and then returns it to the pool. Both pipes
are located near the surface of the water. The flow
rate is 15 gallons per minute. The water entering the
pump is at 0 psig, and the water leaving the pump is
at 10 psig.
A. The diameter of the pipe that leaves the pump is
1 inch. How much flow work is done by the water
as it leaves the pump and enters the pipe?
B. The water returns to the pool through an opening that is 1.5 inches in diameter, located at the
surface of the water, where the pressure is 1 atm.
How much work is done by the water as it leaves
the pipe and enters the pool?
C. “The system” consists of the water in the pump
and in the pipes that transport water between the
pump and the pool. Is the system at steady state,
equilibrium, both, or neither?
1-25. The Reaumur temperature scale, while now obscure,
was once in common use in some parts of the world.
The normal freezing point of water is defined as
0 degrees Reaumur and the normal boiling point of
water is defined as 80 degrees Reaumur. Tolstoy’s
War and Peace mentions the temperature “minus
20 degrees Reaumur.” What is this in Celsius, Fahrenheit, Kelvin and Rankine?
1-26. The furlong is a unit of length, equal to one-eighth
of a mile, which today is most commonly associated
with horse racing. The fathom is a unit of length,
equal to six feet, which is most commonly associated
with measuring the depth of water.
A. The Kentucky Derby is a 10-furlong race. If a
horse finishes in exactly two minutes, what is the
horse’s average velocity in meters per second? In
miles per hour?
41
B. A fishing supply store sells lines that are 20 fathoms long. How many meters is this?
C. A treasure chest weighs 100 lbm and is raised to
the surface from a depth of 500 fathoms. If gravity is the only force opposing the motion, how
much work was required, in Joules?
1-27. Calorie (energy) and horsepower (power) are two
non-SI units that you might see on occasion.
A. Death Valley National Park contains a region
that is rich in borax. In the 19th century, borax
was harvested and shipped on wagons pulled by
20-mule teams. Estimate the power driving these
wagons, in Watts, using the assumption that a
20-mule team provides 20 horsepower.
B. A stove produces 10,000 calories of heat. What is
this in Joules? In BTU?
C. An engine produces 100,000 calories of work in
1 minute. What is the power of the engine in
horsepower, and in Watts?
1-28. Use the data in the steam tables to answer the
following:
A. Find the change in internal energy when 100 kg
of steam at constant pressure P = 1 bar has its
temperature reduced from 300°C to 100°C.
B. Find the change in internal energy when 100 kg of
liquid water at constant pressure P = 200 bar has
its temperature increased from 240°C to 300°C.
C. Energy was transferred from the system in part A,
and energy was transferred to the system in part
B. What form would you expect these energy
transfers took?
D. Your answers to parts A and B should be similar
in magnitude, though different in sign. Would it
be possible to accomplish both of the processes
in parts A and B simultaneously, by taking most
of the energy that was removed from the steam
described in part A and transferring it to the liquid water described in part B?
1-29. A balloon is inflated from a negligible initial volume
to a final volume of 200 cm3. How much work is
done by the balloon on the surroundings if the pressure opposing the expansion is
A. P = 1 bar
B. P = 0.5 bar
C. P = 0 bar
D. P = 3 bar
E. Can you think of locations where each of the
“surroundings” pressures given in parts A–D
would be realistic?
1-30. “The system” is a large ship and everything inside it.
The ship turns off its engines, and is floating in the
ocean. No material enters or leaves the ship during
the process. Some descriptions of this situation are
given below. For each, indicate whether it is true or
false, and explain why.
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42
Fundamentals of Chemical Engineering Thermodynamics
A. The system is not at equilibrium, because equilibrium implies no external forces are acting on
the system and the ship has forces acting upon
it. The system is at a steady state, however, because the forces acting upon it are balanced.
B. The system is not at equilibrium, because the
force of gravity is acting upon the ship but there
is no upward force balancing the downward force
of gravity. However, the system is at a steady
state, because it is not moving.
C. The system is both at equilibrium and at a steady
state, because the ship is not moving, and there
is no driving force for motion: the forces acting
upon it balance each other and there is no driving force for change.
D. The system is neither at equilibrium nor at a steady
state, because no object in the ocean is perfectly
motionless. The ship bobs up and down with the
waves, and likely drifts in a horizontal direction
due to currents. If the position of the system is
changing, it can’t be at steady state or equilibrium.
1-31. “The system” is a large ship and its contents. The inside of the ship and the air outside the ship are at
the same temperature. The ship is sailing north at a
constant speed of 20 knots. The engines are powered
by burning liquid fuel, and the gaseous by-products
(primarily carbon dioxide and water) are vented to
the atmosphere, but nothing else enters or leaves
the system. Some descriptions of this situation are
given below. For each, indicate whether it is true or
false, and explain why.
A. The system is at a steady state, because its velocity is constant. However, it is not at equilibriumthe fact that the ship is moving indicates that the
forces are not balanced.
B. The system is not a at steady state, because the
amount of fuel inside the system is changing.
However, the system is at equilibrium, because
it is at the same temperature as the surroundings;
there is no driving force for heat transfer.
C. The system is neither at equilibrium nor at steady
state.
D. The system is adiabatic, because there is no temperature driving force that would cause heat
transfer to occur.
E. The system is an isolated system, because nothing is entering it and there is no heat transfer.
1-32. You are collecting data from the literature on a compound, for which you need to know the specific internal energy at a number of different states. You’ve
found some data from three different sources, but
they each use different reference states and the units
aren’t uniform either. The data is shown in the table
below. Fill in all of the empty cells in the table, so
that you have correct values of Û for all seven conditions (A–G) at all three reference states.
Û, Source 1
Û, Source 2
Û, Source 3
State
Phase
T (°C)
P (bar)
A
Solid
17
1
B
Liquid
17
1
218.0 J/g
C
Liquid
25
1
0
D
Liquid
82
1
E
Vapor
82
1
67.0 J/g
F
Vapor
100
1.33
136.0 J/g
G
Liquid
100
1.33
37.0 J/g
0
20,470 ft · lbf /lbm
102,970 ft · lbf /lbm
0
1.8 g LO S S A R y O F S yM B O L S
A
area
K.E.
kinetic energy
P.E. potential energy
a
acceleration
l
linear distance, or length
Q
F
force
T
temperature
gravitational constant
M
·
m
mass
g
mass flow rate
t
time
h
height
P
pressure
V
volume
heat
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 1 Introduction
·
V
volumetric flow rate
·
W
power
density
v
velocity
WEC
work of expansion/contraction
angular distance
W
work
WS
shaft work
43
1.9 R E F E R E N C E S
Arce, A.; Martinez-Ageitos, J.; Soto, A., Vapor-Liquid
Equilibria at 101.32 kPa of the Ternary Systems 2Methoxy-2-methylpropane 1 Methanol 1 Water and 2Methoxy-2-methylpropane 1 Ethanol 1 Water, J. Chem.
Eng. Data, 1998, 43, 708–13.
Cantor, G. N.; Gooding, D.; James, F. A. J., Michael Faraday, Humanities Press, Atlantic Highlands, NJ, 1996.
Fandary, M. S. H.; Aljima, A. S.; Al-Kandary, J. A., LiquidLiquid Equilibria for the System Water 1 Ethanol 1
Ethyl tert-Butyl Ether, J. Chem. Eng. Data, 1999, 44,
1129–31.
Olson, R. (editor), Biographical Encyclopedia of Scientists,
Marshall Cavendish, Tarrytown, NY 1998.
Perry, R. H.; Green, D. W., Perry’s Chemical Engineers’
Handbook, 7th ed., McGraw-Hill, New York, NY, 1997.
Rajendran, M.; Renganarayanan, S.; Madhavan, P. R.;
Srinivasan, D., Effect of Dissolved Salts on the Heat of
Mixing of Three Binary Systems, J. Chem. Eng. Data,
1989, 34, 375.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
The Physical Properties
of Pure Compounds
2
LEARNINg OBJECTIVES
This chapter is intended to help you learn how to:
Envision and qualitatively predict the interrelationships among pressure, temperature,
and molar volume of a pure material
Define four states of matter: solid, liquid, vapor, and supercritical fluid
Recognize qualitatively the conditions (primarily temperature and pressure) at which
solids, liquids, vapors, and supercritical fluids exist and at which transitions between
the phases occur
Appreciate the physical significance of the critical point and the triple point
Distinguish between intensive and extensive properties of a material
Recognize state properties of a material, and appreciate the significance of state
properties in solving problems
Define enthalpy in relation to other state properties
Quantify changes in enthalpy and internal energy using heat capacity
List the three attributes of an ideal gas, and recognize situations in which real vapors
are reasonably modeled as ideal gases
Define equation of state
Quantify P, V, and T using the ideal gas law or van der Waals equation of state
T
he first chapter presented an overview of the forms of energy and introduced the
concept that energy cannot be created or destroyed but can be converted from one
form into another. Converting a chemical’s internal energy into a different form (e.g.,
heat, work, or, as in a nozzle, kinetic energy) is one of the recurring themes in the examples
throughout this book. A challenge in modeling or designing such processes comes from
the fact that internal energy cannot be measured directly. Three properties that are measurable—pressure, temperature, and volume—are the primary focus of this chapter. We
discuss the interrelationships among P, V, and T that are observed in pure compounds, and
consider how more abstract properties like internal energy can be related to measurable
properties.
45
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46
Fundamentals of Chemical Engineering Thermodynamics
2.1 MOTIVATIONAL EXAMPLE: Vapor Pressure of Water and Its Effect
on the rankine cycle
Section 1.2.3 described the purpose of the Rankine cycle—conversion of heat into
shaft work—and the role of each of the four steps: boiler, turbine, condenser, and
pump. However, the discussion in Chapter 1 was qualitative. Before a working Rankine cycle actually can be designed and built, a number of specific, quantitative design questions need to be answered, such as the following:
What are the pressures of the water entering and leaving the pump?
What is the temperature of the boiler, and what is the source of its heat?
What is the temperature of the condenser, and how is the cooling accomplished?
How much shaft work must the engine produce?
What flow rate of water/steam is needed to attain the necessary shaft work?
Though we often
think of water only
as a liquid (and use
the word in that way),
water is the name for
the compound H 2O,
whether it is solid,
liquid or vapor.
The relationships
among pressure,
temperature, and
phase for pure
compounds are
discussed further in
Section 2.2.1.
The answers to these questions are rooted in the physical properties of water or
whatever fluid is circulating in the heat engine. To illustrate this, consider the operation
of the turbine. The entire purpose of the Rankine cycle is to produce shaft work. Mechanically, this shaft work comes from the force of the water vapor acting on the blades
of the turbine—the higher the pressure of the entering vapor, the faster the blades will
spin. However, as designers, we cannot simply assign an arbitrary high pressure (e.g., 100
bar) to the steam entering the turbine—we are constrained by the boiling point of water.
For a given temperature, there is a unique pressure above which water will be
liquid and below which it will be vapor. This pressure is called the vapor pressure. Conversely, at any pressure, there is a unique boiling temperature above which water will be
vapor and below which it will be liquid. At P 5 100 bar, this boiling temperature is 311°C.
In the Rankine cycle, the steam entering the turbine comes from a boiler. Its
temperature is constrained by the source of the heat used in the boiler. If the heat
source is at 300°C, then the water/steam in the boiler must be below 300°C, or there
will be no driving force for heat transfer to occur. But if the liquid water entering the
“boiler” is at P 5 100 bar and it isn’t heated to at least 311°C, then it won’t boil at all.
There also may be practical and safety barriers to operating a turbine at such a high
pressure, but the vapor pressure represents a fundamental constraint.
Similarly, the temperature of the condenser is constrained by the available heat
sink. If the coolant in the condenser is simply air or water at ambient temperature (e.g.,
T 5 25°C), the condenser must operate above ambient temperature. At T 5 40°C, for
example, the vapor pressure of water is 0.0738 bar.
If the steam leaving the turbine is at a pressure lower than this, it won’t condense
at 40°C. At pressures below 611.7 Pa (0.006117 bar), the liquid state doesn’t exist at
all; water vapor that is cooled at these pressures will eventually reach a temperature
at which it converts directly into ice. Thus, knowing vapor pressure and its relationship to temperature is of fundamental importance in designing a Rankine heat engine or any other process that involves liquid-vapor transitions.
The aforementioned vapor pressures are available in the “steam tables,” which
are located in Appendix A. The steam tables also contain several other physical
properties relevant in modeling processes like the Rankine Cycle. For example,
Appendix A-3 indicates that for steam at T 5 250°C and P 5 25 bar, the specific
volume is 0.0871 m3/kg, the specific internal energy is 2663.3 kJ/kg, the specific enthalpy is 2880.9 kJ/kg and the specific entropy is 6.4107 kJ/kg ∙ K. Internal energy was
discussed in Section 1.4.6, while enthalpy is introduced in Section 2.3.1 and entropy
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C H A P T E R 2 The Physical Properties of Pure Compounds
is defined in Chapter 4. Volume, internal energy, enthalpy and entropy are all state
properties; their values depend only upon the current state of the substance, and are
independent of how the current state was reached. This fact is what makes tabulation
of data in Appendix A possible; a kilogram of steam at T 5 250°C and P 5 25 bar
always has the same volume and internal energy, regardless of how that temperature
and pressure were reached. State properties are discussed in Section 2.2.3. Note,
too, that all of these properties are expressed “per kilogram” of steam. The volume
of steam (in m3 for example) depends upon how much steam is present. However,
specific volume (m3/kg) is the same at T 5 250°C and P 5 25 bar, regardless of the
amount of steam present. Specific volume is an example of an intensive property,
discussed in detail in Section 2.2.4.
In Section 5.2, we will return to the Rankine cycle a third time. We will apply
energy balances (covered in Chapter 3) and entropy balances (covered in Chapter 4) to make quantitative determinations regarding the Rankine cycle: the rate at
which work is produced in the turbine, the state of the material leaving the turbine,
etc. All of these calculations rely upon the data in the steam tables. Unfortunately,
most chemical compounds have not been studied as extensively as water. Indeed,
one thing chemical engineers do is invent new compounds! Consequently, a major
thrust in thermodynamics is building models that allow one to estimate the physical
properties of chemical compounds from limited data.
Section 2.3 provides an introduction to simple thermodynamic models of pure
compounds.
Chapters 6 through 8 describe modeling strategies that are significantly more
complex—but more broadly applicable. Again, the focus is on pure compounds.
Chapters 9 through 13 describe modeling strategies that are applicable to mixtures of compounds.
2.2 Physical Properties of Pure chemical compounds
Pressure, temperature and volume are three physical properties of matter that are
familiar to us from everyday experience, as are the physical states of matter (e.g.,
solid, liquid, or vapor). Section 2.1 provided one example of the significance of physical properties, where effective operation of the Rankine cycle is dependent upon
H2O changing between the liquid and vapor phases, and these phase transitions are
governed by temperature and pressure. Throughout this book, we will explore many
other properties (e.g., internal energy, enthalpy, entropy, Gibbs energy, and fugacity) which are less tangible than pressure, temperature, and volume but are just as
important in the modeling and designing of chemical processes. A unifying theme in
chemical engineering thermodynamics is
The physical properties of chemical compounds are inherently related to each other.
The practice of thermodynamics consists of exploring and quantifying these interrelationships and applying the insights gained to answer questions, make predictions,
and solve problems.
2.2.1 the P-V-T behavior of real compounds
To illustrate the interrelationships of physical properties of compounds, we consider
the most commonly observed states of matter. Figure 2-1 displays how the state of
matter of a pure compound relates to pressure and temperature. As temperature
increases at a constant pressure (e.g., P1 in the diagram), the compound undergoes
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47
48
Fundamentals of Chemical Engineering Thermodynamics
Supercritical
fluid
Pressure
Critical point
P1
Liquid
Solid
P2
Triple
point
Vapor
Gas
T1
T2
Temperature
Figure 2‑1 Typical phase transition diagram for pure compounds.
the familiar transition from solid to liquid to vapor. At lower pressures (e.g., P2),
no liquid phase occurs. Instead, at a certain temperature, the solid converts directly
into gas, which is a process called sublimation. This is observed when dry ice (which
is solid CO2) sublimates at ambient conditions. Key definitions and observations illustrated in Figure 2-1 are given here.
Section 2.1 illustrated
the significance of
the vapor pressure in
designing a Rankine
heat engine.
For a particular pressure, the temperature at which the solid–liquid transition
occurs is called the melting point or freezing point, the temperature at which the
liquid–vapor transition occurs is called the boiling point, and the temperature at
which the solid–vapor transition occurs is called the sublimation point.
The melting point, boiling point, and sublimation point all increase with increasing pressure.
The melting point and boiling point at atmospheric pressure specifically are
called the normal melting point and normal boiling point.
The pressure at which the liquid–vapor transition or solid–vapor transition
occurs is called the vapor pressure. Vapor pressure increases with increasing
temperature.
There is a unique temperature and pressure at which the solid, liquid, and vapor
states can all exist in equilibrium with each other. This is called the triple point.
There is a unique temperature above which no liquid phase occurs regardless of
pressure. This is called the critical temperature (TC).
There is a unique pressure above which no vapor phase occurs regardless of
temperature. This is called the critical pressure (PC).
A material that is both above the critical pressure and above the critical temperature is called a supercritical fluid.
The triple point temperatures and pressures for different compounds vary
tremendously, as illustrated in Table 2-1. For each individual compound, however,
the triple point is unique. Consequently, the triple point is useful for providing
repeatable temperatures for calibrations. For example, the triple point temperature of ethylene carbonate is 36.315°C, which is a useful reference point for the
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C H A P T E R 2 The Physical Properties of Pure Compounds
Table 2‑1 Comparison of conditions at which phase changes occur for various compounds.
compound
normal
Melting
Point (K)1
normal
boiling
Point (K)1
triple
Point T
(K)2
triple
Point P
(bar)2
critical
temperature Tc (K)1
critical
Pressure
Pc (bar)1
Water
273.15
373.2
273.16
0.0061
647
220.6
Oxygen
54.8
90.2
54.36
0.00152
155
50.4
Nitrogen
63.3
77.4
63.18
0.126
126.2
34.0
Carbon
Dioxide
N/A
N/A
216.55
5.17
304.18
73.80
Methane
85.7
111
90.68
1.17
190.6
46.1
Ammonia
195
238
195.4
0.06076
405.4
113
FOOd FOr
thOught
2-1
Why are the normal
boiling and melting
points for carbon
dioxide listed as “not
applicable?”
Based on data from NIST Webbook (Brown, 2012); Cengel, et. al., 2012
calibration of clinical thermometers because it’s close to human body temperature
(Machin, 2005). As mentioned in Section 1.3.8, the triple point of water is used as a
reference point in defining the Kelvin temperature scale.
The supercritical state is also worth a bit more discussion. Typically, supercritical fluids are transparent and look like gases, but they have properties that are more
commonly associated with liquids, such as high densities. Supercritical fluid extraction is an important application for supercritical fluids. In this process, compounds are
separated by dissolving one or more of them in a supercritical fluid. Carbon dioxide is
the most commonly used solvent. The high densities allow supercritical fluids to act as
solvents, but the process is made easier by the fact that supercritical fluids generally
have transport properties (such as high diffusivities) that are more typical of gases.
As a note on nomenclature, the term “fluids” includes liquids, vapors, gases, and
supercritical fluids. All of these states of matter have flexible macroscopic structures
that take on the shape of their container, rather than retaining their shape as solids do. This “catch-all” term is useful because of similarities in how liquids, gases,
and supercritical fluids are handled. For example, all fluids are easily transported
through pipes, but solids are not. Another note on terminology is that the words
“gas” and “vapor” are often used interchangeably. In this book, the term “gas” is
used to describe a compound that is above its critical temperature but below its
critical pressure, and the term “vapor” is used for a compound that is below its critical temperature but above its boiling temperature, as illustrated in Figure 2-1. Thus,
a vapor condenses if it is isothermally compressed to its vapor pressure (see T1 in
Figure 2-1), while a gas does not condense—no matter how much it is isothermally
compressed (see T2 in Figure 2-1). This distinction is primarily semantic, as our methods of modeling gases and vapors are substantially the same.
The temperatures and pressures at which phase transitions occur vary tremendously from one compound to another, as illustrated in Table 2-1. Qualitatively,
however, almost all compounds exhibit the behavior illustrated in Figure 2-1. Rare
exceptions occur—such as in compounds that have no measurable boiling point
because they decompose at elevated temperatures. In this case the process of heating
converts them into other chemical compounds more readily than it converts them
into vapors. Note that Figure 2-1 specifically describes a pure compound. The phase
behavior of mixtures is much more complex and will be addressed in Chapter 9.
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49
50
Fundamentals of Chemical Engineering Thermodynamics
uid
T 5 Tc
Saturated
liq
Pressure (P)
T . Tc
T , Tc
Sat
ura
ted
vap
or
Increasing temperature
Molar volume (V )
Figure 2‑2 Typical relationships between P, V and T for liquid, vapor, and
supercritical phases in a pure compound. Each line shows the pressure (P)
versus the molar volume ( V ) for a constant temperature (T ).
Beyond the question of which phase will be present at a particular temperature
and pressure, the physical properties of a phase are also dependent upon temperature
and pressure. Figure 2-2 illustrates typical interrelationships among pressure, temperature, and molar volume for the vapor, liquid, and supercritical phases. Combined,
Figures 2-1 and 2-2 give a good overview of the interrelationships among P, V, and
T for a typical compound.
Notable observations and definitions related to Figure 2-2 include the following
statements.
FOOd FOr
thOught
2-2
Are “subcooled liquid”
and “compressed
liquid” synonymous
terms that can be used
interchangeably? Why
or why not?
Vapor molar volume increases with increasing temperature and decreases with
increasing pressure.
Liquid molar volumes also increase with increasing temperature and decrease
with increasing pressure, but the effects of temperature and pressure on liquid
molar volume are generally much less significant than on vapor molar volume.
A vapor that is at a boiling point is called a saturated vapor, and a liquid that is
at a boiling point is called a saturated liquid.
A vapor that is at a temperature above its boiling point is called a superheated vapor.
A liquid that is at a temperature below its boiling point is called a subcooled liquid.
A liquid that is at a pressure above its vapor pressure is called a compressed liquid.
As pressure increases, the molar volumes of the saturated liquids and saturated
vapors at a given pressure become closer together, as illustrated in Figure 2-2. At the
critical point, the liquid and vapor phases become indistinguishable from each other.
At pressures above the critical pressure, there is no longer a boiling point or a
liquid–vapor transition, but supercritical fluids do follow the trend of decreasing
volume with increasing pressure and increasing volume with increasing temperature.
The motivational example (Section 2.1) mentioned several other physical properties
of chemical compounds, including internal energy, enthalpy, and entropy (which will be
discussed in Chapter 4). Quantifying the interdependence of the physical properties of
chemical compounds allows us to solve practical problems like those given here:
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C H A P T E R 2 The Physical Properties of Pure Compounds
Knowledge of both the enthalpy and entropy of the steam entering the turbine
in the Rankine cycle is essential for predicting the work produced, as described
in Section 5.2. Enthalpy is defined in Section 2.3.1 and entropy in Section 4.3.
Many chemical reactions are best carried out at high pressure and/or high temperature, so the reactants need to be compressed and/or heated prior to entering
the reactor. Knowing how internal energy and enthalpy relate to temperature and pressure allows the designer to predict how much heat and work are
required for these processes.
Sections 2.2.3 and 2.2.4 provide a context for defining and comparing physical properties of compounds. Before closing this section, the question “How many
states of matter are there?” is worth mentioning briefly. Solids, liquids, and gases are
familiar in our everyday lives and are sometimes termed “the three states of matter.”
Introductory chemistry and physics books often indicate that there are four states
of matter: solid, liquid, gas, and plasma. Plasma is a highly ionized gaseous state that
occurs at high temperatures. Much of the material in stars is in the plasma state. Here
on Earth, naturally occurring plasma is comparatively rare, although it is produced
by lightning bolts and flames. Plasmas have many practical engineering applications
(e.g., lighting, conduction, and surface cleaning) but do not play a significant role in
mainstream chemical engineering practice.
There is no clear consensus on exactly how many distinct states of matter are
known. While most introductory chemistry and physics books do not describe the
supercritical fluid as a distinct ‘state of matter,’ a plausible argument can be made
that it should be regarded as one. Other esoteric states such as superglass, supersolid,
and Bose-Einstein condensates exist at extreme conditions and are sometimes described as additional “states of matter.”
Regardless of the semantic issue of whether it is or is not a state of matter, the supercritical fluid is certainly important in chemical engineering thermodynamics. The
application of supercritical fluid extraction was mentioned previously. In addition, the
critical point plays a prominent role in our efforts to build mathematical models of
real compounds, as illustrated in Chapter 7. By contrast, plasmas and the various esoteric states of matter mentioned here are not of central importance to chemical engineering thermodynamics, and detailed discussion of them is beyond the scope of this
introductory book. Consequently, through the remainder of this book, solid, liquid,
vapor/gas, and supercritical fluid are the only “states of matter” to be discussed.
FOOd FOr
thOught
2-3
You can recognize
solids, liquids, and
gases from your
everyday experience,
but can you give
concise definitions of
“solid,” “liquid,” and
“gas” that a grade
school student would
understand? Try it.
2.2.2 Forms and sources of Physical Property data
The previous section described the pressure-volume-temperature (or P-V-T ) and
phase behavior of real compounds and underscored the importance of being able to
quantify the relationships between the properties of compounds.
For some compounds, there exists very complete data regarding phase transitions,
P-V-T behavior, and other physical properties (e.g., internal energy) that are of interest.
Appendix A contains various steam tables that provide detailed data for water.
The saturated steam tables show the properties of saturated liquid water
and saturated water vapor at a broad range of temperatures and pressures.
Table A-1 is organized by pressure and Table A-2 is organized by temperature.
Table A-3 gives properties of superheated steam and is organized by pressure. For
each pressure, the physical properties of steam at a variety of temperatures are shown.
Table A-4 is organized similarly to the superheated-steam tables and shows the
properties of compressed liquid water.
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51
Fundamentals of Chemical Engineering Thermodynamics
20
THERMODYNAMIC
PROPERTIES OF
03
0.0
10
8
05
2
/(kg.K
0.
00
0.0
7
0.00
1.5
.25kJ
0.01
1.75
^
S51
6
4
0.01
2
T 5 120K
4
2.25
0.02
2.5
0.03
2
0.05
g.
K)
kJ
/(k
3.
75
3.5
3.2
5
S^
5
06
0.9
x 5 0.8
0.6
0.7
0.5
0.4
0.3
3
line
uid
liq
0.2
0.1
satu
rate
d
0.6
vapor
300
350
400
1
0.8
0.6
0.2
3
saturated
250
^
H, kJ/kg
330
200
300
150
0.14
^ 5 0.3 m
V
4
315
100
270
240
255
210
225
180
195
150
165
120
135
50
285
0.1
T 5105K
0
P, MPa
0.07
2.7
5
105
lin
e
^
V 5 0.0
014 m 3
/kg
)
NITROGEN
0.5
25
.
4 0.7
450
/kg
0.4
0.2
0.1
500
Based on W.C. Reynolds, Thermodynamic Properties in SI, Department of Mechanical Engineering,
Stanford University, 1979
52
Figure 2‑3 Thermodynamic diagram for nitrogen.
Appendix B-1 reviews interpolation, which can be used to estimate data when
the exact number needed is not in the tables (e.g., V is needed at T 5 2608C and
is known at T 5 2508C and T 5 2758C).
“Specific” properties
(e.g., specific enthalpy
and specific volume)
are defined in Section
2.2.3.
Data analogous to that given in the steam tables is often consolidated into a
figure. Figure 2-3, for example, shows physical property data for nitrogen. The axes
of Figure 2-3 are pressure versus specific enthalpy, but the figure also contains temperature, specific entropy, and specific volume. If any two of these five properties are
known, the other three can be found. Comparable diagrams for several other compounds are provided in Appendix F. More extensive data is available in sources such
as Perry’s Chemical Engineers Handbook (Green, 2007), The Properties of Gases
and Liquids (Poling, 2001), and the NIST Chemistry WebBook (available at time of
writing at http://webbook.nist.gov/chemistry/).
Overall, relatively few chemical compounds have been studied in the same level
of detail as water or nitrogen. Chemical engineers are routinely required to estimate
physical properties of compounds and/or mixtures when the relevant data is unavailable. Much of this book is devoted to constructing mathematical models that are
used to produce such estimates.
2.2.3 state Properties and Path-dependent Properties
A state property is one that describes the condition of a material or system at a
particular time and is independent of how the system or material reached that condition. Without the idea of a state property, it would be impossible to make something
like the steam tables or Figure 2-3; we could not tabulate a value for the specific
internal energy of water at 100°C and 10 bar unless it was always the same for water
at 100°C and 10 bar.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 2 The Physical Properties of Pure Compounds
2 feet
1 foot
In
1 foot
Out
2 feet
Figure 2‑4 The current location of the ball is
“the exact center of the maze,” and this is true
regardless of the path taken to reach the location.
Consider the ball in the maze in Figure 2-4. We can describe the ball’s position
in different ways.
It is in the exact center of the maze—equidistant from the four corners.
It is one foot below the entrance.
It is one foot to the left of the exit.
There are many paths by which it could have reached this point. Those paths
have different lengths, and it would take the ball different amounts of time to follow
those paths from the entrance. However, these bulleted facts are true regardless of
the path taken.
Similarly, the current state of a material can be described in many different
ways: There is a specific amount of the material (typically quantified through mass
or moles), and it has a specific pressure, temperature, and volume. These properties
are independent of the path taken to reach the current state. Consider the ice/water
mixture in Figure 2-5. Assuming the mixture is well mixed and at atmospheric pressure, its temperature must be 08C (328F); this is the only temperature at which water
can exist in both liquid and solid forms in equilibrium at atmospheric pressure. Five
minutes ago, it may have been all ice at 258C, all liquid water at 5°C, or any one of
innumerable other possibilities, but the current temperature of 08C is independent
of the path taken to reach the current state.
The significance of state properties in thermodynamics comes from the fact that
they are intrinsically related to each other. Some useful relationships between state
properties include:
The data provided in the steam tables, and similar tables and charts for other
compounds.
The formula of Mass 5 Density 3 Volume (M 5 rV).
The relationship between height and potential energy (Equation 1.30).
Ice
P 5 1 atm
T 5 08C
Freezing
Melting
Liquid water
P 5 1 atm
T 5 08C
Figure 2‑5 An icewater mixture in
equilibrium at P 5 1
atm must have a
temperature of 0°C.
The relationship between velocity and kinetic energy (Equation 1.29).
The ideal gas law and other equations of state (discussed in Section 2.3.3).
Numerous additional expressions to be developed later in the book, particularly
in Chapter 6.
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53
54
Fundamentals of Chemical Engineering Thermodynamics
Examples 4-2 and 4-3
will give an illustration
of two very different
paths from the same
initial state to the
same final state.
Specific internal energy
is an example of an
intensive property,
which is discussed
further in the next
section.
Like the properties they relate, these equations and relationships can be applied to the state of a system at any time—independent of how that state came
about.
We can also imagine path-dependent properties. Returning to Figure 2-4, if
asked “How many times has the ball crossed this spot?” or “How far has the
ball travelled since it first entered the maze?” the answer clearly depends upon
the path, while the answer to “How far is the ball from the left-hand edge of the
maze right now?” is only dependent upon the current location of the ball—not
on the path taken to reach that location. Thus, the total distance travelled is a
path-dependent property and the current position is a state property. The primary path-dependent quantities that will concern us in thermodynamics are heat
and work. For example, the steam tables tell us that the specific internal energy of
steam is Û 5 2663.3 kJ/kg at T 5 2508C and P 5 25 bar. There are many ways in
which this steam can be expanded and cooled into a saturated liquid at T 5 408C.
The specific internal energy of the water is Û 5 167.5 kJ/kg at the end of the
process—regardless of the path. Û is a state function. Thus, the steam experienced
a change in internal energy of 22495.8 kJ/kg regardless of the path. But how
much of this energy was transferred in the form of heat, and how much in the
form of work? Many different answers to this question exist, but heat and work
depend upon the specific path.
The symbol Δ represents the change in a state property. Using volume as an
example, the change in volume of a system during a process (ΔV) is
DV 5 Vfinal 2 Vinitial
(2.1)
An analogous definition can be made for any state property. (Example 1-8 used ΔT
to represent the change in the temperature of water.)
A common careless error is inverting the order of the terms (e.g., computing
ΔP as Pinitial 2 Pfinal). To help avoid such confusion, in this book we use the symbol Δ in discussions and in posing questions (e.g., “find ΔU”), but in performing
the actual calculations we generally avoid this shorthand (e.g., “Ufinal 2 Uinitial” or
“U2 2 U1”).
2.2.4 intensive and extensive Material Properties
In chemical engineering thermodynamics, much of our interest is in predicting how
one state property (such as volume) of a material will be influenced by changes
inanother state property (such as pressure). In modeling the many physical properties of a material, there is an important distinction between intensive and extensive
properties.
The word “extensive”
shares a root with the
word “extend.” You
might say that as the
amount of a material
extends (gets larger)
the extensive properties change, but the
intensive properties
do not.
An intensive property of a system is not dependent upon either the size of the
system or the amount of the material present in the system.
An extensive property of a system is a property that is proportional to the
amount of material present in the system.
For example, the density of liquid water at 20°C and 1 bar is 0.998 g/cm3 (Table 2-2
shows the density of water at a variety of conditions). This value of the density is
valid whether examining a drop of water or a reservoir full of water: thus, density is
an example of an intensive property. Conversely, the mass and volume of water are
examples of extensive properties; they are directly proportional to the number of
moles of water present.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 2 The Physical Properties of Pure Compounds
55
Table 2‑2 Density of liquid water in g/cm3 at various temperatures and pressures.
P 5 1 bar
P 5 10 bar
P 5 100 bar
P 5 1000 bar
T 5 208C
0.998
0.999
1.003
1.040
T 5 408C
0.992
0.993
0.997
1.032
T 5 608C
0.983
0.984
0.987
1.023
T 5 808C
0.972
0.972
0.976
1.012
T 5 1008C
0.958*
0.959
0.963
1.000
*This is actually the density of liquid water at 99.68C, which is the boiling point at P 5 1 bar. At T 5 100.08C and
P 5 1.00 bar, pure water is a vapor.
The distinction between intensive and extensive properties is important,
because intensive properties are fundamental and repeatable properties of a material, while extensive properties only can be used to describe a particular sample of
a material. For example, if told “the mass of the water is 40 grams,” the value of 40
grams is an extensive property that is applicable only to that particular sample of
water. But “the density of water at T 5 50°C and P 5 1 bar is 0.988 g/cm3” is an intensive property and is valid for any system or process involving water at that T and P.
The relationship among the temperature, pressure, and density of pure liquid
water is illustrated in Table 2-2. Notice that, if any two of these properties are known,
there is only one value the third can have, and it can be determined from the table.
This table illustrates a general principle:
For a pure substance in a single homogeneous phase, if two intensive properties have
been fixed, all other intensive properties must have a unique value.
Because temperature and pressure are relatively easy to measure and control,
all intensive properties are generally thought of as being related to temperature and
pressure. We often refer to the density, specific volume, specific internal energy, or
specific enthalpy of a compound at a particular T and P. Conceptually, however,
we can set any two intensive properties. If the specific volume of water is V̂ 5
0.001043 m3/kg and the specific internal energy is Û 5 418.8 kJ/kg, there is no freedom for also setting the temperature and pressure; they must be T 5 100°C and
P 5 10 bar. This is one of the data points given in Appendix A-4.
Because of these fundamental relationships between intensive properties, it
is usually convenient or necessary to focus on intensive properties, so it is useful
to define intensive counterparts for all extensive properties. For example, volume
is an extensive property, but one frequently refers to the specific volume or the
molar volume. The specific volume is the volume of a material per unit mass, and
the molar volume is the volume per mole of material, as summarized in Table 2-3.
Table 2-4 provides a number of prominent examples of intensive and extensive
properties.
If the substance is
pure but is in two
phases in equilibrium
(e.g., vapor–liquid
equilibrium), you can
only “set” one intensive variable.
Throughout this book,
the hat (circumflex)
signifies a property
expressed on a mass
basis (e.g., specific
internal energy Û) and
the underline signifies a
property expressed on a
molar basis (e.g., molar
internal energy U).
Table 2‑3 Common units for volume, specific volume, and molar volume.
Property
symbol
typical units
Volume
V
m3, L, cm3, ft3
Specific Volume
V̂
m3/kg, L/kg, ft3/lbm
Molar Volume
V
m3/mol, cm3/mol, ft3/lb-mole
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56
Fundamentals of Chemical Engineering Thermodynamics
Table 2‑4 Examples of common intensive and extensive properties.
intensive Property: symbol and si units
The fact that the
water is at its boiling
point doesn’t guarantee that two phases
are present. Water
at 100°C and 1 atm
can exist as saturated
liquid, as saturated
vapor, or as a vapor–
liquid equilibrium
(VLE) mixture.
extensive Property: symbol and si units
Specific Volume 5 V̂
m3/kg
Volume 5 V
m3
Density 5 ρ
kg/m3
Mass 5 M
kg
Specific Enthalpy 5 Ĥ
J/kg
Enthalpy 5 H
J
Specific Internal Energy 5 Û
J/kg
Internal Energy 5 U
J
Specific Entropy 5 Ŝ
J/kg · K
Entropy 5 S
J/K
Temperature 5 T
K
Kinetic Energy 5 K.E.
J
Pressure 5 P
Pa
Potential energy 5 P.E.
J
2.2.5 state Properties of Multiphase Mixtures
Section 2.2.3 introduced the concept of a state property and Section 2.2.4 discussed
intensive and extensive properties. This section examines applying these concepts to
systems that, like the ice-water mixture in Figure 2-5, contain more than one phase.
In Section 2.2.4, we noted that if two intensive properties have been specified for a pure
compound in a single, homogeneous phase, all others are constrained to unique values.
A more general statement of the interrelationship between intensive properties is
given by the gibbs phase rule:
F5C212
The Gibbs phase rule
was proposed by
Josiah Willard Gibbs
in 1878. (Olson, 1998)
FOOd FOr
thOught
2-4
If C 5 1 and π 5 3,
then F 5 0. How is
this fact reflected in
Figure 2-1?
(2.2)
where F is the number of degrees of freedom in the system, C is the number of distinct chemical compounds in the system, and π is the number of distinct phases in the
system.
The number of degrees of freedom (F ) is the number of intensive variables that can be established before all others are constrained to unique values.
Degrees of freedom are important in design applications; you might think of
the number of degrees of freedom as the number of independent decisions you
are able to make regarding particular process conditions. For example, with two
degrees of freedom you can decide what the temperature and pressure of a gas
will be, but you can’t choose a third intensive variable, such as molar volume, as
only one molar volume is physically attainable at a particular temperature and
pressure.
The number of distinct chemical compounds in the system is C. The first several
chapters of this book focus on pure compounds (C 5 1), but starting in Chapter 9,
we will examine mixtures. Adding compounds introduces more degrees of freedom,
and the mole fraction of a particular compound is an intensive property that is commonly used to satisfy these degrees of freedom.
The number of distinct phases in the system is π. Pure liquid water is a simple
system: There is a single compound (C 5 1) and a single phase (π 5 1). Therefore,
there are two degrees of freedom, as illustrated in Table 2-2. However, if pure liquid
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C H A P T E R 2 The Physical Properties of Pure Compounds
57
water and ice are present in equilibrium, as in Figure 2-5, then C 5 1, but there are
two phases (π 5 2). The simultaneous presence of liquid and solid means that the
mixture must be at the melting point, a fact that accounts for one of the degrees of
freedom and leaves only one degree remaining (F 5 1).
In examining multiphase systems, we must recognize that each individual phase
can be described by a set of state properties, and in addition, there are state properties that describe the system as a whole. Consider water at its boiling point at P 5 1
bar. According to the steam tables, saturated liquid water at P 5 1 bar has a specific
internal energy of 417.4 kJ/kg. This is a state property; it is always true for saturated
liquid water at P 5 1 bar—regardless of how the water arrived at that state, of the
quantity of liquid water present, and of whether there is or is not a vapor phase
present in equilibrium with the liquid. Similarly, saturated water vapor at P 5 1 bar
has a specific internal energy of 2505.6 kJ/kg—regardless of whether the saturated
vapor is a single homogeneous phase or is in equilibrium with a liquid phase. We can
also compute the specific internal energy of a liquid–vapor mixture as a whole, as
illustrated in Example 2-1.
a vaPOr–Liquid equiLibriuM (vLe) Mixture in a rigid cOntainer
examPle 2‑1
Five kilograms of water is contained in a rigid, 3 m3 vessel at P 5 1 bar.
A. What is the temperature and physical state (liquid, vapor, or both liquid and vapor)
of the water?
B. What is the specific internal energy of the water in the vessel?
SOLUTION A:
Step 1 Identify intensive properties
We know from the Gibbs phase rule that it takes two intensive properties to specify the
state of a pure, homogeneous phase. Here one intensive property is given as P 5 1 bar.
A second intensive property can be inferred from the known mass and volume:
V̂ 5
V
3 m3
m3
5
5 0.6
M 5 kg
kg
(2.3)
Our first instinct is to consult the steam tables and find the temperature where saturated
liquid, saturated vapor or superheated vapor has a specific volume of 0.6 m3/kg. But there
is no such temperature. Saturated vapor has V̂V 5 1.6919 m3/kg, and superheated steam
at P 5 1 bar has even higher specific volumes. There is also no circumstance in which
liquid water at P 5 1 bar has V̂ 5 0.6 m3/kg; the specific volume of saturated liquid water
is only V̂L 5 0.001043 m3/kg. The only state in which water can have a pressure P 5 1
bar and a specific volume 0.6 m3/kg is when it is in vapor–liquid equilibrium.
Liquid or vapor
3 m3
3 m3
1 bar
1 bar
1 phase
Liquid–vapor
equilibrium (VLE)
According to the
Gibbs phase rule,
F 5 2 when P 5 1 and
C 5 1.
The container is a
closed system; no
material is entering or
leaving. Individually,
the liquid or vapor
phase is an open system, since material can
move between them.
2 phases
Figure 2‑6 Possible states for water in a 3 m3 vessel at P 5 1 bar. In
Example 2-1, we determine which is observed in reality.
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58
Fundamentals of Chemical Engineering Thermodynamics
According to the Gibbs phase rule, there is only one degree of freedom (F 5 1) for
a single component (C 5 1) in a two-phase mixture (π 5 2). The pressure P 5 1 bar fills
this one degree. The only possible temperature is the boiling point T 5 99.6°C. Similarly,
all intensive properties of the saturated liquid and vapor phases individually (V̂, Û, etc.)
have fixed values, which also can be obtained from the steam tables.
We have partially answered the question by determining the temperature and demonstrating that there is a vapor–liquid equilibrium mixture. To fully characterize the state
of the material, we will quantify the amounts of liquid and vapor.
Step 2 Quantify phases within VLE mixture
The total volume of the mixture is the sum of the liquid and vapor volumes:
V 5 VL 1 VV
(2.4)
V 5 MLV̂L 1 MVV̂V
This is an isochoric
system; the volume is
constant. Notice the
problem statement
indicated the vessel
was “rigid.”
The total volume V is a constant and is given. While the specific volumes of the liquid
and vapor phases are known from step 1, this is one equation in two unknowns (ML and
MV). The known total mass is the second constraint:
M 5 ML 1 MV
ML 5 M 2 MV
(2.5)
Substituting Equation 2.5 into Equation 2.4 gives
V 5 sM 2 MVdV̂L 1 MVV̂V
1
3 m3 5 s5 kg 2 MVd 0.001043
3
2
(2.6)
1
3
m
m
1 MV 1.6919
kg
kg
2
MV 5 1.77 kg
Solving Equation 2.6 gives ML 5 3.23 kg.
SOLUTION B:
Step 3 Determine specific internal energy of system
The total internal energy of the system is the sum of the internal energies of the individual phases.
U 5 UL 1 UV 5 MLÛL 1 MVÛV
Some books define
“quality” as the liquid
fraction (rather than
the vapor fraction) in
a VLE mixture.
ML/M 5 (M 2 MV)/M
512q
q represents both the
mass fraction of vapor
and the mole fraction
of vapor. Since the
liquid and vapor are
the same compound
they have the same
molecular mass.
(2.7)
The specific internal energy of a system comprised of two phases is defined the same way
as the specific internal energy of a single phase. This is the internal energy divided by the
mass:
Û 5
MLÛL 1 MVÛV
M
(2.8)
For a vapor–liquid system, the quality (q) is the mass or mole fraction of a system that is
in the vapor phase. Here the quality is
q5
MV 1.77 kg
5
5 0.354
M
5 kg
(2.9)
Inserting this into Equation 2.8 gives
Û 5 s1 2 qdÛL 1 qÛV
1
Û 5 s1 2 0.354d 417.4
2
1
(2.10)
2
kJ
kJ
kJ
1 s0.354d 2505.6
5 1156.6
kg
kg
kg
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C H A P T E R 2 The Physical Properties of Pure Compounds
Example 2-1 introduced the quality(q) of a system in vapor–liquid equilibrium, which is
the fraction of mass that is in the vapor phase. It also illustrated how intensive properties
representing a multiphase equilibrium system can be computed using a weighted average of the intensive properties of the individual phases. If a system consists of liquid and
vapor in equilibrium, an intensive property of the overall system can be computed as
M 5 s1 2 qdML 1 qMV
(2.11)
where M can represent any intensive property (e.g., specific enthalpy or molar
entropy) and q represents the quality.
2.3 thermodynamic Models of Physical Properties
Pressure, volume, and temperature are easily measured properties of materials that
have great practical significance in both engineering and thermodynamics. Section 1.4.6
described internal energy, which cannot be measured directly but is a fundamental
quantity in chemical engineering thermodynamics. Sections 2.2.1 and 2.2.2 outlined
the importance of quantifying the interrelationships between various physical properties of compounds, but they also noted that the relevant data is not always available.
This section begins the process of developing mathematical models that quantify the
interrelationships among the key thermodynamic properties of materials.
2.3.1 enthalpy
Enthalpy is another state property that quantifies energy. Like internal energy, it
cannot be measured directly and cannot be known in an absolute sense, but it is
quantified relative to a reference state. This section introduces and explores how
enthalpy is useful in solving problems.
Recall that the rate of flow work is equal to
·
·
· PV̂
W 5 PV 5 m
(2.12)
For phases in equilibrium, temperature
and pressure of the
phases—the “liquid
T,” “vapor T,” and
“system T ”—are
all identical. Other
intensive properties
of the system like
specific volume, specific internal energy,
specific enthalpy, and
specific entropy can
be determined from an
equation analogous to
Equation 2.11.
Flow work per
kilogram of a fluid is
equal to PV̂, as demonstrated in Section 1.4.3. Thus, Û 1 PV̂
can be considered
“internal energy
plus flow work” for
a stream of material
that is flowing.
Physically, internal
energy is always
“significant” in that
all chemical streams
contain a significant
amount of internal
energy. Mathematically, however, internal
energy is sometimes
unchanged in a
process and therefore
may not be “significant” in the solution of
a particular problem.
Marafona/Shutterstock.com
A chemical plant is shown in Figure 2-7. One immediate observation is that the
chemical plant contains hundreds of pipes, so liquids and gases are continuously moving
throughout the plant. When material flows through a pipe, flow work is always occurring. The material flowing through the pipe also has stored energy. Potential and kinetic
energy may or may not be significant, depending upon height of the pipe and velocity
of the fluid, but internal energy is always significant. Consequently, in accounting for the
energy in any process that involves moving streams, the sum U 1 PV consistently occurs,
where U represents the internal energy in a stream and PV quantifies the flow work
done by the stream.
59
Figure 2‑7 A chemical plant.
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60
Fundamentals of Chemical Engineering Thermodynamics
The Greek word “thalpein” means “to warm.”
The sum is defined as enthalpy:
H 5 U 1 PV
(2.13)
where H is enthalpy, U is internal energy, P is pressure, and V is volume.
Internal energy is
quantified relative to
a reference state, as
discussed in Section 1.4.6. When converting between U and H
using Equation 2.13,
take care that both
are expressed relative
to the same reference
conditions.
Note that the U and PV terms both have units of energy (e.g., Joules). Thus,
enthalpy also has units of energy. Furthermore, note that U, P, and V are all state
properties. Since H is computed exclusively from state properties, it also must be a
state property. This illustrates a general principle used several times throughout this
book: Any property that is defined as a mathematical function calculated entirely from
state properties is itself a state property.
For example, mass is a state property. Dividing both sides of Equation 2.13 by
the mass gives
Ĥ 5 Û 1 PV̂
(2.14)
Ĥ is the specific enthalpy and is also a state property. Unlike H, Ĥ is also an intensive
property, because Û, P, and V̂ are all intensive properties. We can divide both sides of
Equation 2.13 by the number of moles, giving the molar enthalpy as
H = U + PV
(2.15)
Again, molar enthalpy is an intensive property as well as a state property.
2.3.2 heat capacity
The number 4.18 J/g · K
is specific to liquid
water and cannot
be applied to ice or
steam. Furthermore, it
is only an approximation for liquid water
at most conditions, as
heat capacity is not a
constant with respect
to temperature and
pressure.
Internal energy, heat, and temperature were described in Sections 1.3.6 through 1.3.8.
It was noted that temperature is not precisely a measurement of internal energy,
but temperature and internal energy are related to each other. Heat capacity is the
property used to quantify that relationship. Heat capacity is described as the amount
of energy required to increase the temperature of a unit of material by one degree
(any T scale can be used). The unit of material can be either moles or a mass unit. For
example, as taught in introductory chemistry or physics, the heat capacity of liquid
water is about 4.18 J/g · K. That is, it takes 4.18 Joule of heat to raise the temperature
of one gram of water by one degree Kelvin (or one degree Celsius). However, this
definition of heat capacity isn’t very clear. To explore why, consider this question:
How would you experimentally measure the heat capacity of helium gas?
One possibility is shown in Figure 2-8. If a vessel is filled with a known mass of
helium (e.g., 10 grams), a known amount of heat is added (e.g., 30 Joules), and
T
10 g of helium
Electric
heat
source
Figure 2‑8 Schematic of a rigid (fixed
volume) calorimeter system for measuring
heat capacity of helium.
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C H A P T E R 2 The Physical Properties of Pure Compounds
the temperature change is accurately measured (e.g., initially 20.00°C, at the end
20.97°C), the heat capacity is
Heat Capacity 5
30 Joules
J
5 3.1
s10 gramsds20.97 2 20.008Cd
g8C
(2.16)
However, heating a substance in a sealed, rigid container is generally regarded
as unsafe because of the potential for pressure build-up, resulting in rupture or
explosion. Instead, picture a system like the one in Figure 2-9, where a piston is
used to maintain a constant applied pressure while allowing the volume to change.
At first glance, this looks like a slightly different way to measure exactly the same
property. But it produces a different answer. This time, the same 30 Joules of heat
are added to the same 10 grams of helium, but the temperature only increases to
20.58°C. Thus,
Heat Capacity 5
30 Joules
J
5 5.2
s10 gramsds20.58 2 20.008Cd
g8C
61
This result, to two
significant digits, is the
correct ideal gas heat
capacity CV of helium
gas. CV is defined in
this section, and the
concept of an ideal
gas heat capacity is
discussed in Section 2.3.3.
(2.17)
Patm
T
10 g of helium
Electric
heat
source
Figure 2‑9 Schematic of a
constant-pressure, variable volume
calorimeter system for measuring
the heat capacity of helium.
Why are the answers in Equations 2.16 and 2.17 different? Because there is
more going on in these processes than just changes in temperature. When helium
was heated in a closed, rigid vessel, both the temperature and the pressure
increased (though only the temperature change was monitored). When the same
gas was heated in a constant-pressure vessel, the temperature and volume both
increased.
Which is the “right” heat capacity? Both are based on real data obtained from
well-designed experiments and both are “right.” The problem is that “the heat
required to raise the temperature by one degree” is not a satisfactory definition of
heat capacity; the answer depends upon how the experiment is conducted. The challenge arises from the fact that temperature, pressure, volume, and internal energy are
all interrelated in ways that are often complex and subtle.
Consequently, we need formal mathematical definitions of heat capacity—
definitions that explicitly account for the fact that internal energy is not only a function of temperature. The two heat capacities in common use are given here.
FOOd FOr
thOught
2-5
Can you propose an
explanation for why
the constant-pressure
heat-capacity measurement is larger
than the constantvolume heat-capacity
measurement?
Remember that “Q”
is not a state function,
but is path-dependent.
We have to specify
how we added the
heat (constant molar
volume or constant
pressure).
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62
Fundamentals of Chemical Engineering Thermodynamics
The constant-volume heat capacity (CV) is the partial derivative of molar internal
energy (U) with respect to temperature (T ) at constant molar volume (V ). Thus,
Heat capacity is
defined here on a
molar basis, but it also
can be defined on a
mass basis as
−Û
ĈV 5
−T V̂
CV 5
(2.18)
V
The constant-pressure heat capacity (CP) is the partial derivative of molar enthalpy
(H) with respect to temperature (T ) at constant pressure (P). So,
1 2
Some chemical
engineering programs
include a physical
chemistry course as
either an elective
or a requirement. A
course like this provides a deeper insight
into the relationships
between molecular
structure and physical
properties.
1−U−T 2
Cp 5
1 −T 2
−H
(2.19)
P
Example 2-2 helps illustrate the rationale for these mathematical definitions and
how the heat capacity can be used. The question of “why do different compounds
have different heat capacities?” is discussed here. When 30 Joules of energy is added
to 10 grams of helium, the helium increases in temperature by almost a degree.
(Remember, the actual temperature increase depends on how we added the energy.)
But what form does this added energy take? In general, molecules can store kinetic
energy in the forms of translational, vibrational, or rotational motion (see Figure
2-10). Helium is a monatomic gas. Because there are no chemical bonds, there is no
freedom for vibrational or rotational motion. Thus, the entire 30 Joules takes the
form of added translational kinetic energy, which is the motion of the entire molecule. Larger molecules with more vibrational and/or rotational degrees of freedom
have higher heat capacities, because they can store energy in other forms in addition
to translation. Thus, the more non-translational modes in a compound, the larger the
heat capacity and the less the temperature of the compound will increase for a given
amount of a heat added.
Translational:
V
Vibrational:
Rotational:
Figure 2‑10 Two atoms (shown as spheres) joined
by a single bond, and the three forms their molecular
kinetic energy can take.
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C H A P T E R 2 The Physical Properties of Pure Compounds
Liquid heat caPacity
A liquid has Cˆ V of 2.15 J/g · K. What is the change in specific internal energy when this
liquid is heated from 295 to 300 K?
^
U2
T1 295 K
^
U1 ?
T2 300 K
Figure 2‑11 Schematic of process
examined in Example 2-2.
SOLUTION: Intuitively, you might look at this question and say “It takes 2.15 J to increase
the temperature by one degree, so the answer is 2.15 3 5.” For this example, that is a satisfactory answer. However, the following is a more rigorous mathematical solution with a
discussion of the potential shortcomings of the “intuitive” approach.
63
examPle 2‑2
While finding Û2 2 Û1
by itself may not seem
like a particularly
interesting calculation,
Chapter 3 covers energy balances, which
quantify relationships
among internal
energy, work, and heat.
Work and heat have
more tangible significance than Û; they are
resources that have
monetary value.
Step 1 Relate internal energy to specific volume and temperature
For a pure, single-phase compound, the Gibbs phase rule (introduced in Section 2.2.5)
indicates that there are two degrees of freedom. This means that two intensive variables
are enough to completely describe the state of the material (e.g., if the temperature and
specific volume are known, there is only one value the specific internal energy can have).
Furthermore, changes in specific internal energy can be related to changes in temperature and specific volume using partial differential calculus:
dÛ 5
1 −T 2 dT 1 1 −V̂ 2 dV̂
−Û
−Û
(2.20)
T
V̂
While many students initially find partial derivative expressions like this one intimidating, they are readily placed in a physical context.
−Û
dT represents a small change in the temperature, and
or Cˆ V quantifies the
−T V̂
Chapter 6 focuses on
writing and solving
partial derivative
expressions like this
one.
1 2
effect of this small change in temperature on the specific internal energy.
dV̂ represents a small change in specific volume, and
quantifies the effect of
1−Û
−V̂ 2
T
this small change in specific volume on the internal energy.
Summing the small changes (integrating) yields the total change in specific internal
energy for a process.
Step 2 Simplifying assumption
Liquids and solids typically expand with increasing temperature, but the change in volume is often comparatively small, as illustrated in Table 2-1. Since this example examines
a small temperature interval, it is logical to assume that the volume of the liquid is essentially unchanged, and the dV̂ term is negligible. Stated mathematically, if volume is unchanged, the limits of integration on the dV̂ term are identical, which means the integral
−Û
equals zero regardless of what
is. With this simplification, Equation 2.20 becomes
−V̂ T
1 2
dÛ 5
dT 5Ĉ dT
1−Û
−T 2
Vˆ
V
(2.21)
FOOd FOr
thOught
2-6
You’d like to quantify
the expansion of water
vapor with increasing temperature and
compare it to the
expansion of liquid
water with increasing
temperature. What’s
the best way to make
this comparison using
data from the steam
tables?
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64
Fundamentals of Chemical Engineering Thermodynamics
Heat capacity expressions account for the
changes in energy that
accompany changes
in temperature. If the
liquid boiled between
295 and 300 K, these
equations wouldn’t
be adequate—we also
would have to account
for the change in specific internal energy
on vaporization.
Step 3 Integration
Because ĈV is constant, Equation 2.21 integrates to
1
Û2 2 Û1 5 ĈVsT2 2 T1d 5 2.15
2
J
J
s300 2 295 Kd 5 10.75
g
g?K
DISCUSSION: This is the same answer obtained from the “intuitive” approach, but having analyzed the situation mathematically, we recognize that this answer relies upon the
assumption of specific volume being constant.
Example 2-2 demonstrates the use of the constant volume heat capacity. A key result
is Equation 2.21:
dÛ 5 ĈV dT
But the example reveals that the derivation of Equation 2.21 requires assuming that
specific volume is constant. Many scenarios where such an assumption would not be valid
can be imagined.
What if the problem had concerned a gas, rather than a liquid? Then we certainly
couldn’t assume volume was constant unless the gas was confined in a rigid vessel.
Thermal expansion of liquids tends to become more significant near the critical
point. Since this example did not specify a pressure or even identify the liquid, we
don’t know if it is near the critical point.
What if the temperature increase had been 100 K instead of 5 K? Would it still be
reasonable to assume the change in volume was negligible over this much larger
temperature range?
If neglecting the change in volume isn’t realistic, we cannot complete the calculation
−Û
unless we are able to evaluate
. Chapter 6 discusses methods of evaluating partial
−V̂ T
derivative expressions like this one to solve such problems.
Note that the constant pressure heat capacity can be applied using an equation analogous to 2.21:
1 2
dĤ 5 ĈP dT
(2.22)
But the derivation of Equation 2.22 requires assuming that pressure is constant.
2.3.3 ideal gases
An ideal gas is a hypothetical gas in which the molecules have no intermolecular
interactions and no volume. This section explores the utility of this hypothetical gas
state.
The ideal gas law is undoubtedly familiar to most readers. It is expressed as
PV 5 RT
In practice volume is
always expressed on
an absolute scale, but
pressure and temperature are not.
(2.23)
where P is pressure, V is molar volume, T is temperature, and R is the gas constant.
P, V , and T must be expressed on an absolute scale. The ideal gas law is also frequently written in terms of the total volume:
PV 5 NRT
(2.24)
where N is the number of moles of gas and V is the total volume.
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C H A P T E R 2 The Physical Properties of Pure Compounds
65
It can be proved (K. J. Laidler, 2003) that this expression holds for any gas that
has the following identifying characteristics.
1.
2.
3.
The gas molecules have zero volume.
The particles are in constant motion, and collisions are perfectly elastic.
There are no intermolecular attractions or repulsions between the molecules.
The derivation of the ideal gas model from a starting point of these three assumptions has been called the “kinetic theory of the ideal gas.”
We started this section by calling an ideal gas “hypothetical,” because descriptors like “molecules have zero volume” and “no intermolecular attractions” are never
literally true for real gases. However, there are many practical situations where the
ideal gas law provides an accurate and useful model. Knowing the three characteristics of the ideal gas listed above will help us identify situations in which the ideal gas
model is a reasonable approximation. The following example compares properties
computed for steam using the ideal gas law with data from the steam tables.
assessing accuracy OF ideaL gas LaW
examPle 2‑3
Using the ideal gas law, estimate the specific volume of steam (in L/kg) at each of the following temperatures and pressures, and compare to the specific volume in the steam tables.
A.
B.
C.
D.
T 5 100°C, P 5 0.1 bar
T 5 150°C, P 5 1 bar
T 5 200°C, P 5 10 bar
T 5 300°C, P 5 50 bar
SOLUTION:
Step 1 Determine molar volume from ideal gas law
The ideal gas law, as written in Equation 2.23, can be solved for molar volume as
V5
RT
P
(2.25)
Substitute the known values for case A, recognizing that T must be expressed on an
absolute scale.
cm
1L
s373.15 Kd1
183.14 bar
mol K 2
1000 cm 2
3
3
V5
0.1 bar
5 310.2
L
mol
(2.26)
Step 2 Convert to specific volume
Converting the answer from step 1 into specific volume allows direct comparison to the
data in the superheated steam tables (Appendix A-3). Thus,
1
V̂ 5 310.2
L
mol
1 mol
L
5 17,221
2118.015
g 21 kg 2
kg
1000 g
PitFaLL
PreventiOn
Forgetting to convert
from Celsius to Kelvin
(or Fahrenheit to
Rankine) is a common error in using the
ideal gas law. There
are many equations
throughout this book
in which temperature
MUST be expressed
on an absolute scale.
(2.27)
Cases B, C, and D can be solved in a similar manner. The results are summarized in
Table 2-5.
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66
Fundamentals of Chemical Engineering Thermodynamics
tabLe 2-5 Comparison of actual V̂ for water vapor to those obtained from the ideal gas law.
temperature
Pressure
V̂ (ideal gas
Law)
V̂ (steam
tables)
% difference
100°C
0.1 bar
17,221 L/kg
17,196 L/kg
0.15%
150°C
1 bar
1953 L/kg
1936 L/kg
0.87%
200°C
10 bar
218.4 L/kg
206 L/kg
6.0%
300°C
50 bar
52.90 L/kg
45.32 L/kg
16.7%
Example 2-3 illustrates that the departures from ideal gas behavior tend to increase
with increasing pressure. This observation can be rationalized by considering the three
characteristics of ideal gases mentioned below:
1. The gas molecules have zero volume. All molecules have finite volume, but in
gases at low pressure, the volume of the molecules is not significant compared to
the volumes of the spaces between the molecules. As pressure increases, volume
decreases, and Figure 2-12 illustrates the increasingly large fraction of the total
volume occupied by actual particles.
(a)
(b)
Figure 2‑12 (a) Four gas “molecules” in a large volume. (b) The
same four “molecules” compressed into a smaller volume.
2. Collisions are perfectly elastic. Physically, pressure (P) reflects collisions between
gas molecules and the walls of the container. The kinetic theory of the ideal gas
assumes these collisions are perfectly elastic. Real gas molecules collide not only
with the walls but also with each other. Intermolecular collisions aren’t necessarily
elastic. In real gases, collisions can lead directly to associations and chemical reactions, which in turn affect the pressure and other measurable properties of the gas.
However, as volume increases, whether or not these intermolecular collisions are
elastic becomes less and less important simply because they become less common.
3. There are no intermolecular attractions or repulsions between the molecules.
Water is an extremely polar compound. Consequently, this sounds like quite an
unreasonable description for water. There are definitely strong intermolecular
attractions and repulsions between the dipoles in water molecules. However, the
example implies that water is well approximated by the ideal gas law for pressures below atmospheric. Why should this be?
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C H A P T E R 2 The Physical Properties of Pure Compounds
Coulomb’s law states that the force between two charged particles is directly proportional to the product of their charges and inversely proportional to
the square of the distance between them. Thus,
q1 q2
F 5 ke 2
(2.28)
r
where ke is a constant, q1 and q2 are the charges of the particles, and r is the distance between the particles. Consequently, regardless of how strong the charges,
the force approaches zero as the intermolecular distance gets very large. This
helps explain why even the strongly polar compound of water behaves almost
like an ideal gas at low pressures (and correspondingly at high volumes).
These characteristics are summarized here.
At low pressure, the total volume is large, and the volumes of the particles themselves are insignificant compared to the total volume.
At low pressure, it does not matter much if the collisions are perfectly elastic;
high volumes mean relatively infrequent collisions.
At low pressure, it does not matter much if the particles are attracted to each other
or not; they are generally too far apart for the attractions (or repulsions) to matter.
In thermodynamics, the concept of “ideal gas behavior” has implications far
beyond relating pressures, temperatures, and volumes to each other using the ideal
gas law. For example, consider the molar internal energy of a gas (U). We saw that U
increases with increasing T, and the constant-volume heat capacity CV can be used
to quantify this effect. In general, U is also a function of volume (or, conversely, pressure.) However, for an ideal gas, U is dependent upon temperature—and only temperature. This statement is demonstrated mathematically with techniques covered in
Chapter 6. For now, it will be rationalized as given here.
67
Coulomb’s Law
is named after the
French physicist
Charles Augustin de
Coulomb. (Olson,
1998)
Coulomb’s Law models the charged particles as “point charges,”
which is a good model
if the particle size is
small compared to the
distance between the
particles. Section 7.4.4
describes a model for
molecular interaction
that is useful when the
molecules are very
close together.
Intensive properties
such as molar internal
energy U were introduced in Section 2.2.3.
Internal energy is comprised of microscopic potential and kinetic energy—the
potential and kinetic energy of individual molecules.
Ideal gases have no intermolecular interactions. Therefore, moving the molecules farther apart or closer together has no effect on the energy of each individual molecule.
Consequently, changing the pressure or volume of an ideal gas would not be
expected to affect its molar internal energy (U ).
Recall that CV is defined as
CV 5
1 −T 2
−U
V
Example 2-2 showed that, for constant-volume processes, we can equate
dU 5 CV dT
(2.29)
However, in an ideal gas, volume does not influence U at all, so it does not matter whether volume is constant. Equation 2.29 is always valid for ideal gases. Note
that Cv can be expressed on either a mass or a molar basis. If it is expressed on a mass
basis, Equation 2.29 would read as
dÛ 5 ĈV dT
Equations 2.29 and
2.30 are valid for
constant-volume
processes and for any
process involving an
ideal gas.
(2.30)
Recall that the molar enthalpy is given by
H 5 U 1 PV
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68
Fundamentals of Chemical Engineering Thermodynamics
For an ideal gas, however, the PV term can be equated to RT. So
H 5 U 1 RT
(2.31)
Note the right-hand side, where we have established that U is a function of
temperature—and only temperature. R is a constant, so RT is also a function of only
temperature.
Equation 2.32 is valid
for any constantpressure process and
for any process involving an ideal gas.
“Example 6-9 revisited” gives a mathematical proof of the
fact that Equation 2.32
is always valid for
ideal gases, even if P
isn’t constant.
examPle 2‑4
At time of writing,
the three Goodyear
blimps each hold
202,700 ft3 of helium,
weigh 12,840 lbm with
no lifting gas, and
typically fly at heights
of 1000 to 1500 feet
(obtained from the
Goodyear Blimp
official web site in
October 2013).
Consequently, ideal gas molar enthalpy is also only a function of temperature, and the
equation
dH 5 Cp dT
(2.32)
is valid for any constant-pressure process and for any ideal gas process, regardless of
whether the pressure is constant.
When a vapor is not modeled as an ideal gas, these simplifications do not occur;
U and H for real gases are (or at least, can be) dependent upon pressure and volume
as well as temperature. Consequently, throughout the rest of this book, symbols C *P
and C *V are used for heat capacity values that specifically describe the ideal gas state
to distinguish them from values that are valid in general.
a heLiuM baLLOOn
The balloon portion of a dirigible is filled with 5000 moles of helium (Figure 2-13). Initially, the helium is at P 5 1 atm and T 5 25°C. As the dirigible gains altitude, the pressure drops to P 5 0.95 atm, and the temperature drops to T 5 15°C. Find the changes in
volume, internal energy, and enthalpy for the helium in this process, assuming helium has
a constant ideal gas heat capacity of C *V 5 (1.5)R.
N 5 5000 moles of He; P2 5 0.95 atm; T2 5 158C
N 5 5000 moles of He; P1 5 1 atm; T1 5 258C
Figure 2‑13 Schematic of system in Example 2-4.
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C H A P T E R 2 The Physical Properties of Pure Compounds
69
SOLUTION:
Step 1 Define system
The helium inside the balloon is the system.
Step 2 Obtain volume from ideal gas law
The ideal gas law can be solved for volume where
V5
NRT
P
(2.33)
Because pressure is expressed in atmospheres, the most logical value of R (at least in
computing the initial and final volume) is R 5 0.08206 L · atm/mol · K. Thus,
1
s5000 mold 0.08206
V1 5
2
L ? atm
s298.15 Kd
mol ? K
1 atm
1
s5000 mold 0.08206
V2 5
2
L ? atm
s288.15 Kd
mol ? K
0.95 atm
5 122,300 L
(2.34)
5 124,500 L
(2.35)
Therefore, the volume expands by 2200 L, as the drop in pressure is more significant
than the drop in temperature.
Step 3 Evaluate physical situation
If we assume helium is an ideal gas, the changes in volume and pressure have no effect on
the internal energy and enthalpy. Is such an assumption reasonable? Example 2-3 suggested that the ideal gas model is a reasonable approximation for water vapor at pressures up to 1 atm. Since one of the ideal gas criteria is “no intermolecular interactions,”
we’d expect a non-polar compound like helium to be closer to ideal gas behavior than
water vapor at the same temperature and pressure. Since pressure is at most one atmosphere in this example, we can assume (quite logically) that helium acts like an ideal gas.
FOOd FOr
thOught
2-7
This is a closed system;
no helium is entering
or leaving. Are any
of the other system
descriptors covered in
Section 1.3.1 correct
for this system?
Step 4 Apply heat capacity CV to find change in internal energy
Equation 2.29 is valid for any ideal gas process:
dU 5 C*V dT
Because the given C *V is constant, step 4 simply integrates to
3
U 2 2 U 1 5 RsT2 2 T1d
(2.36)
2
Since the total number of moles is known, multiply both sides of the equation by N to
give the total change in internal energy, rather than the change in molar internal energy. Thus,
U2 2 U1 5 s5000 mold
123218.314 molJ ? K2s288.15 2 298.15 Kd 5 2623,550 J (2.37)
Step 5 Relate enthalpy to internal energy
The change in enthalpy is not as straightforward to determine. We know that, for an ideal
gas, Equation 2.32 is valid, regardless of whether the pressure is constant. Thus,
dH 5 C P* dT
(2.38)
*
P
However, no value of C is given. What other approach to computing the change in
enthalpy is possible? Since we’ve been successful at quantifying the change in internal
energy, it makes sense to apply the definition of enthalpy, which relates H to U as
H 5 U 1 PV
(2.39)
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70
Fundamentals of Chemical Engineering Thermodynamics
Notice that heat capacity, when expressed on
a molar basis, has the
same units as R. While
heat capacity also can
be expressed on a mass
basis, an ideal gas heat
capacity is most commonly expressed on a
molar basis.
or dividing both sides by the number of moles gives
H 5 U 1 PV
(2.40)
This definition can be inserted directly into Equation 2.38 to give
dsU 1 PVd 5 C *P dT
(2.41)
Step 6 Apply ideal gas law
For an ideal gas, PV 5 RT, so Equation 2.41 becomes
dsU 1 RTd 5 C *P dT
(2.42)
*
P
dU 1 RdT 5 C dT
Introducing Equation 2.29 to relate internal energy to temperature gives
C *V dT 1 R dT 5 C *P dT
Note that in Step 6, we
have assumed nothing
other than ideal gas behavior. Thus, Equation
2.44 is always valid for
ideal gases.
(2.43)
Comparing the left- and right-hand sides yields an important property of ideal gases:
C *P 5 C *V 1 R
(2.44)
Step 7 Calculate enthalpy
In this problem, Equation 2.44 is useful because C*V is known to be (1.5)R; thus, C *P 5
(2.5)R. Now that C *P is known to be a constant, it is possible to integrate Equation 2.38 to
5
H 2 2 H 1 5 RsT2 2 T1d
2
(2.45)
Multiplying both sides by the number of moles to get total enthalpy, rather than
specific enthalpy, gives
H2 2 H1 5
1
2
5
J
8.314
s288.15 2 298.15 Kd 5 2 1,039,250 J
2
mol ? K
(2.46)
What if we were doing an example for which using the ideal gas model is NOT a
reasonable approximation (e.g., because the pressure was higher)? Chapter 6 explores
how to quantify the effects of pressure and/or volume on U and H when the ideal gas
approximation does not apply.
FOOd FOr
thOught
2-8
When heat capacity is
expressed on a molar
basis, all monatomic
ideal gases have the
same CV*. Why should
this be?
examPle 2‑5
Example 2-4 illustrates two significant properties of ideal gases:
1. CP* 5 CV* 1 R for all ideal gases. In Example 2-4, C *V was a constant, while in
Example 2-5 it is a function of temperature, but in all cases C *P 5 C *V 1 R.
2. The given value of C *V 5 (1.5)R is not specific to helium; one result of the Kinetic
Theory of the Ideal Gas is that for any monatomic ideal gas, C *V 5 (1.5)R.
Modeling C *P as constant with respect to temperature is a simplifying approximation that is quite reasonable over small temperature intervals (DT 5 210°C in
Example 2-4) but becomes suspect over larger temperature intervals. Example 2-5
illustrates a more realistic modeling of C *P.
internaL energy OF nitrOgen
The ideal gas heat capacity of nitrogen varies with temperature; it is C *P 5 29.42 2
(2.170 3 1023)T 1 (0.0582 3 1025)T 2 1 (1.305 3 1028)T 3 2 (0.823 3 10211)T 4, with T
equal to the temperature in Kelvin and CP* is in J/mol · K.
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C H A P T E R 2 The Physical Properties of Pure Compounds
A. How much internal energy (per mole) must be added to nitrogen to increase its
temperature from 450 K to 500 K at low pressure?
B.
Repeat part A for an initial temperature of 273 K and a final temperature of 1073 K.
C. The Ideal gas heat capacity of nitrogen is sometimes roughly modeled as C *P 5 (7/2)R.
How different would the answers be if you used that constant value for parts A and B?
SOLUTION A:
Step 1 Assess physical situation
The pressure is not known exactly, but “at low pressure” implies that we can assume
nitrogen acts like an ideal gas. Recall that for an ideal gas, changes in internal energy are
given by Equation 2.29:
71
The heat capacity
data in Appendix D is
tabulated in the form
“CP*/R 5 ,” so that you
can readily express CP
in preferred units by
selecting a value for R.
Here the expression
in Appendix D
was multiplied by
R 5 8.314 J/mol · K.
dU 5 C *V dT
Step 2 Determine C *V
In this case, C *V is not given, but C *P is. It was proved in Example 2-4 that for an ideal gas
C *P 5 C *V 1 R.
C *V 5 C *P 2 R 5 29.42 2 s0.00217dT
(2.47)
1 s5.82 3 1027dT 2 2 s1.305 3 1028dT 3
2 s8.23 3 10212dT 4 2 8.314
CV* 5 21.106 2 s0.00217dT 1 s5.82 3 1027dT 2
2 s1.305 3 1028dT 3 2 s8.23 3 10212dT 4
Step 3 Integrate
Inserting the value of C *V into Equation 2.29 gives
U2 2 U1 5
#
T5500 K
21.106 2 s0.00217dT
(2.48)
T5450 K
1 s5.82 3 1027dT 2 2 s1.305 3 1028dT 3
2s8.23 3 10212dT 4 dT
T2
U 2 2 U 1 5 21.106T 2 s0.00217d
2
3
T
T4
T 5 T5500 K
1 s5.82 3 1027d
2 s1.305 3 1028d
2 s8.23 3 10212d
3
4
5 T5450 K
*
U 2 2 U 1 5 1059
J
mol
SOLUTION B:
Step 4 Compare to part A
The only difference between part B and part A is the limits of integration for Equation 2.48 are now T1 5 273 K and T2 5 1073 K. The result is
3
U 2 2 U 1 5 21.106T 2 s0.00217d
2 s8.23 3 10212d
T2
T3
T4
1 s5.82 3 1027d
2 s1.305 3 1028d
2
3
4
4
T 5 T51073 K
5 T5273 K
U 2 2 U 1 5 17,920
J
mol
If a chemical reaction
is highly exothermic,
the released heat can
lead to unsafe
temperature rises.
Diluting the reactants
with a carrier gas such
as nitrogen is one
way of controlling the
resulting temperature
increase. In this
example, we calculate
the amount of energy
required to produce a
particular temperature
increase.
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72
Fundamentals of Chemical Engineering Thermodynamics
SOLUTION C:
Step 5 Compare to part A
As it was in part A, our starting point for calculating the change in U is
dU 5 C *V dT
This is a property of ideal gases that is valid regardless of the value of Cp* or C*V. Also,
C *p 5 C *V 1 R is a property of ideal gases, which is valid whether Cp* is constant or not.
Thus, if C *p 5 (7/2)R, then
7
5
(2.49)
C *V 5 C *P 2 R 5 R 2 R 5 R
2
2
Step 6 Integrate
Because CV here is considered a constant, the integration of Equation 2.29 simplifies to
U2 2 U1 5
#
T5500 K 5
T5450 K 2
1
R dT
(2.50)
2
J
5
5
J
U 2 2 U 1 5 RsT2 2 T1d 5
8.314
s500 2 450 Kd 5 1039
2
2
mol ? K
mol
or, for the conditions in part B,
1
2
5
J
J
5
8.314
s1073 2 273 Kd 5 16, 628
U 2 2 U 1 5 RsT2 2 T1d 5
2
2
mol . K
mol
For the relatively small temperature interval of 50 K, the answer is only ~2% different
when a constant heat capacity is assumed. For the larger temperature interval of 800 K,
the answers are 7.2% different.
Example 2-5 illustrates that, while internal energy of an ideal gas is solely a function
of temperature, it’s not usually a linear function. The heat capacity is itself dependent on temperature. There is no theoretical reason to expect that C *p will follow the
form C *p 5 A 1 BT 1 CT 2 1 DT 3 1 ET 4; it is simply an empirical formula that works
well. Appendix D gives values for C *p in this form for a number of compounds.
They are called “equations of state” because
they relate state functions to each other.
FOOd FOr
thOught
2-9
“Data trumps theory”
is a popular expression
in science and engineering. Do you agree
with this statement?
2.3.4 equations of state
Example 2-3 used H2O to illustrate the fact that real gases behave like ideal gases
at low pressure, and suggested that P ≤ 1 atm is not a bad general guideline for
what constitutes “low.” The example likely seems contrived in that there’s no obvious practical need to use the ideal gas approximation for water; there is data readily
available and one can simply look up the volume, internal energy, and enthalpy at
most any conditions. However, chemical engineers routinely work with thousands of
different compounds, very few of which have been studied as extensively as water.
Consequently, while an engineer should certainly make use of reliable data when
available, it is extremely valuable to have available strategies for the accurate prediction of unknown properties for chemical compounds. Equations of state are at the
heart of such strategies.
Formally, an equation of state is a relationship among temperature, pressure, and
molar volume, such that if any two are known, the third can be calculated.
The ideal gas law:
P5
RT
V
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C H A P T E R 2 The Physical Properties of Pure Compounds
73
fits the definition. If any two of P, V , and T are known, the third can be found, as was
done in Example 2-3. Thus the ideal gas law is an equation of state that has practical
value at low pressures. However, at elevated pressures, real gases exhibit substantial departure from ideal gas behavior. Many more complex equations of state have
been proposed to quantify P, V , and T at a broader range of conditions.
Among the oldest of these is the van der Waals equation:
P5
RT
a
2
V 2 b V2
(2.51)
where a and b are constants that have unique values for each compound and R is
the gas constant. Temperature (T ), pressure (P), and molar volume (V ) must all be expressed on an absolute scale.
Physical significances of a and b can be considered as
“b” is a parameter that has the units of specific volume and accounts for
the volume of the particles. According to the ideal gas law, as P approaches
infinity, V approaches zero. But V 5 0 is an unrealistic limit because real
particles have finite volume. If we picture molecules as a collection of rigid
spheres, increasing pressure moves the spheres closer together as illustrated
in Figure 2-12, but once the spheres are packed together a minimum volume has been reached. The parameter b represents this minimum volume;
according to the van der Waals equation, as pressure approaches infinity, V
approaches not 0 but b.
“a” is a parameter that accounts for the effect of intermolecular attractions on
pressure. Consider that if two molecules are attracted to each other, they might
“stick together” and act like one particle. This consolidation of particles would
tend to decrease the pressure exerted by the gas, and thus the sign of the second
term is negative. This term is proportional to 1/V 2 because as V gets smaller, the
particles are closer together and the effect of intermolecular interactions (“a”)
becomes more significant.
While qualitative, these explanations of “a” and “b” allow us to rationalize the
van der Waals equation compared to the ideal gas law.
The ideal gas model assumes particles have zero volume; the van der Waals parameter “b” accounts for real gas departures from this assumption.
The ideal gas model assumes particles have no intermolecular interactions; the
van der Waals parameter “a” accounts for the fact that they do interact.
When V is very large, the a/V 2 term will become vanishingly small, and b will be insignificant compared to V. Thus, as V becomes larger, the van der Waals equation
becomes closer and closer to P 5 RT/V, which is the ideal gas law. This observation is logical in that high volume, and correspondingly low pressure, is the exact
condition under which real gases are expected to behave like ideal gases.
At this point, we introduce the compressibility factor (Z)
Z5
PV
(2.52)
RT
Notice that Z 5 1 for an ideal gas (PV 5 RT). Equations of state are frequently
expressed in terms of Z. Doing this makes comparisons to ideal gas behavior easier;
Formally, b is called
the “excluded volume,”
because if a particle is
occupying space it
“excludes” other particles from that space.
FOOd FOr
thOught
2-10
If a rigid sphere had
a volume of 1 cm3,
would you guess its
“excluded volume”
was 1 cm3, or would
it be some other
number?
Section 7.2.5 discusses
how numerical values
of a and b can be assigned for specific
compounds.
Note that if a 5 b 5 0,
the van der Waals
equation simplifies
to the ideal gas law:
RT
P5
V
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74
Fundamentals of Chemical Engineering Thermodynamics
if a gas has V 5 0.02 m3/mol this number might not hold much insight for you, but
if Z 5 0.65 you know immediately that the gas departs significantly from ideal gas
behavior. The van der Waals EOS, written in terms of Z, is
Z5
V
V2b
2
a
RTV
(2.53)
We close this section with an example illustrating the accuracy of the van der
Waals equation compared to the ideal gas law.
examPle 2‑6
Example 7-4 illustrates the process by
which these values of
the parameters a and
b were calculated.
cOMParing the van der WaaLs equatiOn tO reaL data
For H2O, the following parameters have been proposed for the van der Waals equation
of state: a 5 0.5542 Pa · m6/mol2 and b 5 3.051 3 1025 m3/mol. Find the specific volume of
water at the same temperatures and pressures given in Example 2-3.
SOLUTION:
Step 1 Manipulate van der Waals equation
A complication in solving the van der Waals equation is that it gives the pressure explicitly as
P5
RT
a
2
V 2 b V2
As a result, it is simple to solve when V and T are given and P is unknown, but less
straightforward when V is unknown. Furthermore, if we multiply through the denominators, we have
PsV 2 bd sV 2d 5 RTV 2 2 a sV 2 bd
Many modern calculators are capable of
solving cubic equations.
(2.54)
We can now see more clearly that this is a cubic equation; the left-hand side is of
third degree in V. Appendix B outlines methods for solving cubic equations, so here we
will simply give numerical answers. A source of confusion, however, is that cubic equations
can have three real solutions.
% Error
Step 2 Physical interpretation of result
For the first case, T 5 100°C and P 5 0.1 bar, and when the three solutions for V are converted to specific volume, the results are V̂ 5 2.17 L/kg, V̂ 5 7.76 L/kg, and V̂ 5 17,213 L/kg.
Which one is “right?” Mathematically, all three are “right,” but physically, there can only be
one value of V̂ for steam at a particular T and P. If you look at the ideal gas calculations in
Example 2-3, you can probably deduce that V̂ 5 17,213 L/kg is the physically realistic value;
20%
18%
16%
14%
12%
10%
8%
6%
4%
2%
0%
Ideal Gas
van der Waals
50 bar, 573 K
10 bar, 473 K
1 bar, 423 K
0.1 bar, 373 K
Figure 2‑14 Accuracy of V predicted by ideal gas law versus van der
Waals EOS for steam.
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C H A P T E R 2 The Physical Properties of Pure Compounds
75
the others are much too small for the specific volume of a low-pressure gas. This would be
correct; in using a cubic equation of state, when there are three solutions for V, the largest
value corresponds to the molar volume of the vapor. Cubic equations of state can be developed such that the lowest value of V models the liquid molar volume. This is discussed further in Section 7.2. For some values of T and P, there is only one real solution for V̂; the other
two roots of the equation are imaginary numbers. These are discussed further in Section 7.2.
The V̂ (van der Waals) shown in Table 2-6 is the largest of the solutions forV̂ at each
temperature and pressure. Notice that at P 5 1 bar, use of the ideal gas law is justified; it is
accurate to within 1%, and the more complex van der Waals equation is only fractionally
better. However, as pressure increases, we find that the van der Waals equation models
the actual data for steam much more closely than does the ideal gas law. Chapter 7 covers
modern equations of state, which can be used to model a wide range of compounds with
a still greater degree of accuracy than that shown for the van der Waals equation.
tabLe 2-6 Comparison of actual specific volumes of water vapor to calculations using ideal
gas and van der Waals equations of state.
Pressure
(bar)
V̂ (steam
tables)
L/kg
V̂ (van der
Waals)
(L/kg)
% error,
van der
Waals
V̂ (ideal
gas law)
(L/kg)
% error,
ideal gas
law
100
0.1
17,196
17,213
0.10
17,221
0.15
150
1
1936
1946
0.52
1953
0.87
200
10
206
212.1
2.96
218.4
6.00
300
50
45.32
47.7
5.25
52.90
16.70
temp.
(°c)
2.3.5 simple Models of Liquids and solids
Up to this point only modeling of the vapor phase has been discussed. Chapter 7 discusses the use of equations of state to describe liquids; indeed, it shows how a single
equation of state can be used to quantify the molar volumes of a substance in both the
liquid and the vapor phases. Generally, however, the volumes of liquids and solids are
much less sensitive to changes in pressure and temperatures than the volumes of vapors
and gases. Because of this observation, both liquids and solids are frequently modeled as
having constant volume (especially over small ranges of temperature and/or pressure).
V L < constant
(2.55)
V S < constant
(2.56)
FOOd FOr
thOught
2-11
Are Equations 2.55
and 2.56 equations of
state?
If a material (liquid or solid) is modeled as having constant volume, internal
energy can be calculated as shown in Example 2-2, where
dU 5 CV dT
(2.57)
Further, because V is much smaller in solids and liquids than in vapors and gases, at low
pressures PV is usually very small compared to U. Consequently, for solids and liquids at
low pressures, it is often reasonable to approximate enthalpy and internal energy as equal:
H 5 U 1 PV < U
(2.58)
This means that the constant pressure and constant volume heat capacities are
approximately equal:
CV < CP
(2.59)
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76
Fundamentals of Chemical Engineering Thermodynamics
2.3.6 summary of simple thermodynamic Models for Materials
Table 2-7 summarizes the properties of ideal gases that are of particular utility in
solving thermodynamics problems. Section 2.3.5 summarizes some ways to model
thermodynamic properties for liquids and solids, based on the assumption that volume is constant with respect to both pressure and temperature. While simplified,
these models are adequate for many practical problems. Chapters 3 through 5 will
focus on thermodynamic analysis of systems for which either comprehensive data
(e.g., the steam tables) are available, or else these simple models are sufficient. Chapter 6 outlines the limitations of the simple models, discusses more complex models
capable of describing real compounds at a broader range of conditions, and illustrates how accurate equations of state play a pivotal role in such models. A more
complete discussion of equations of state is then presented in Chapter 7.
tabLe 2-7 Properties of ideal gases.
Property
equation
comment
P, V , and T
PV 5 RT
The ideal gas law
U
dU 5 C *V dT
Internal energy for an ideal gas is dependent only
upon temperature
H
dH 5 C *P dT
Enthalpy for an ideal gas is dependent only upon
temperature
Heat capacity
C *P 5 C *V 1 R
Valid whether C *V is modeled as constant or not
2.4 s u M M a r y O F c h a P t e r t W O
A state property is a property that depends upon the state of the system, as
described by other state properties, but does not depend upon the path taken to
arrive at the current state.
Temperature, pressure, volume, and internal energy are all state properties.
Heat and work are NOT state properties.
Any property such as enthalpy, which is defined as a mathematical function of
other state properties (H 5 U 1 PV), is also a state property.
State properties of materials are related to each other. Degrees of freedom are
the number of independent intensive state properties that must be determined
before all other intensive state properties have unique values.
Degrees of freedom can be quantified using the Gibbs phase rule:
F5C212
A pure material in a single phase has two degrees of freedom.
An intensive property of a material does not depend upon the quantity of the
material, while an extensive property does depend upon the quantity of the
material present.
An equation of state is an equation that quantifies the interrelationships
among pressure, temperature, and molar volume of a material.
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C H A P T E R 2 The Physical Properties of Pure Compounds
77
The ideal gas law is the simplest equation of state for gases.
Real vapors and gases behave like ideal gases at low pressures, and Table 2-7
summarizes several useful properties of ideal gases.
The heat capacity of a material is the amount of energy required to raise the temperature of a set amount of the material (e.g., one mole, one gram) by one degree.
Heat capacity can be defined through either a constant-volume heat capacity
(CV) or a constant-pressure heat capacity (CP).
2.5 e x e r c i s e s
2-1.
Consider the P-T diagram in Figure 2-15 for a pure
substance:
2-4.
Read the following paragraph.
A plane was scheduled to fly a round trip from an
airport to a rural landing strip. The mass of the plane,
when empty, was 100,000 lbs. It was loaded with
10,000 gallons of fuel. The fuel weighed 6 pounds
per gallon. The plane took off from its home airport,
and made a detour to take aerial photographs, so it
flew 220 miles before landing at the landing strip,
which is 150 miles from the airport. There were
4600 gallons of fuel remaining in the plane when it
landed at the landing strip.
A. For every number in the paragraph, indicate
whether or not the number represents a state
property.
B. For every number in the paragraph, excluding
the distances, indicate whether the number represents an intensive or extensive property.
2-5.
Read the following paragraph.
A room has a volume of 1200 ft3. The initial temperature was 678F. The room contained 40 kilograms
of air, and 20.9 mol% of it oxygen. A space heater
was accidentally turned on. The space heater added
1000 kJ of heat to the room, increasing the temperature by 238F.
D
B
C
A pure compound is initially in the liquid phase.
Each of the following processes ends with the compound in a single, pure phase that is not liquid.
Identify the phase that will exist at the end of each
process. If there is more than one possibility, give all
the possibilities.
A. The pressure is increased isothermally.
B. The pressure is decreased isothermally.
C. The temperature is increased isobarically.
D. The temperature is decreased isobarically.
E. Temperature and pressure are increased simultaneously.
F. Temperature and pressure are decreased simultaneously.
G. Temperature is increased while pressure is
decreased.
H. Temperature is decreased while pressure is
increased.
E
A
P
2-3.
F
T
Figure 2‑15 Phase diagram for a typical compound.
A. What phase is represented by point A? According to the Gibbs phase rule, how many degrees of
freedom exist at this point?
B. What phase is point B? How many degrees of
freedom exist at this point?
C. How many phases are in equilibrium at point
C? What is the name of this point? How many
degrees of freedom exist at this point?
D. How many phases are in equilibrium at point
D? What is the name of this point? How many
degrees of freedom exist at this point?
E. What phase is point E?
F. What phase is point F?
G. What is the physical significance of the curve that
exists between points C and D?
2-2.
A pure compound is initially in the vapor phase, at
its vapor pressure. The pressure is increased isothermally. Which phase will exist at the end of the process? For each of the options listed below, determine
whether or not it is possible.
A. Solid
B. Liquid
C. Solid–liquid equilibrium mixture
D. Supercritical fluid
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78
Fundamentals of Chemical Engineering Thermodynamics
A. For every number in the paragraph, indicate
whether the number represents a state property.
B. For every state property, indicate whether the
number represents an intensive or extensive
property.
2-6.
2-7.
Most (though not all) of the people who climb
Mt. Everest do so with the aid of oxygen masks and
tanks. This is because the barometric pressure at the
peak of Mt. Everest is about one third that at sea
level. Estimate the number of moles of oxygen in one
cubic meter of air, assuming air is an ideal gas that is
21 mol% oxygen:
A. P 5 1 bar and T 5 258C
B. P 5 0.35 bar and T 5 2108C
A 0.5 m3 container holds ideal gas, initially at T 5 50 8C
and P 5 1 bar. The ideal gas has CV* 5 2.5R.
A. How many moles of ideal gas are in the container?
B. What is the change in internal energy of the gas
when it is heated from 50 to 758C?
C. What is the change in enthalpy of the gas when it
is heated from 50 to 758C?
2-8.
If the heat capacity of an ideal gas is C *P 5 30 1 0.05T,
with C *P in J/mol · K and T representing the temperature in K, what is the change in internal energy
and the change in enthalpy for five moles of this gas
when its heated from P 5 1 bar and T 5 300 K to
P 5 5 bar and T 5 600 K?
2-9.
A quantity of carbon dioxide is confined in a sealed
container. For each of the following cases, estimate
the pressure of the carbon dioxide, using both the
ideal gas law and the van der Waals equation of state.
The parameters used to model carbon dioxide using
the van der Waals equation of state are a 5 3.658 3
106 bar · cm6/mol and b 5 42.86 cm3/mol.
A. T 5 500 K, V 5 50 L, N 5 1 mol
B. T 5 500 K, V 5 50 L, N 5 5 mol
C. T 5 500 K, V 5 50 L, M 5 1 kg
D. T 5 750 K, V 5 50 L, M 5 1 kg
E. T 5 750 K, V 5 15 L, N 5 1 mol
F. T 5 750 K, V 5 1 L, N 5 1 mol
2.6 P r O b L e M s
2-10. The boiler is an important unit operation in the
Rankine cycle. This problem further explores the
phenomenon of “boiling.”
A. When you are heating water on your stove, before the water reaches 1008C, you see little bubbles of gas forming. What is that, and why does
that happen?
B. Is it possible to make water boil at below 1008C?
If so, how?
C. What is the difference between “evaporation”
and “boiling?”
2-11. 10 mol/s of gas flow through a turbine. Find the
change in enthalpy that the gas experiences:
A. The gas is steam, with an inlet temperature and
pressure T 5 6008C and P 5 10 bar, and an outlet
temperature and pressure T 5 4008C and P 5 1
bar. Use the steam tables.
B. The gas is steam, with the same inlet and outlet
conditions as in part A. Model the steam as an ideal
gas using the value of C *P given in Appendix D.
C. The gas is nitrogen, with an inlet temperature
and pressure of T 5 300 K and P 5 10 bar, and
an outlet temperature and pressure T 5 200 K
and P 5 1 bar. Use Figure 2-3.
D. The gas is nitrogen with the same inlet and outlet conditions as in part C. Model the nitrogen
as an ideal gas using the value of C *P given in
Appendix D.
E. Compare the answers to parts A and B and to
parts C and D. Comment on whether they are significantly different from each other, and if so, why.
2-12. Using data from the steam tables in Appendix
A, estimate the constant pressure heat capacity
of superheated steam at 350 kPa, 2008C and at
700 kPa, 2008C. Are the answers very different
from each other? (NOTE: You may need to use
the “limit” definition of the derivative to help you
get started.)
2-13. The specific enthalpy of liquid water at typical ambient conditions, like T 5 258C and P 5 1 bar, is
not given in the steam tables. However, the specific
enthalpy of saturated liquid at P 5 1 bar is given.
A. Using the approximation that Cˆ P for liquid water
is constant at 4.19 J/g · K, estimate the specific
enthalpy of liquid water at T 5 258C and P 5 1
bar.
B. Compare the answer you obtained in part A to
the specific enthalpy of saturated liquid water at
T 5 258C.
2-14. This problem is an expansion of Example 2-3. The
table below lists 10 sets of conditions—five temperatures at a constant P, and five pressures at a constant T. For each T and P, find:
The specific volume of steam, from the steam
tables
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C H A P T E R 2 The Physical Properties of Pure Compounds
The specific volume of steam, from the ideal gas law
Comment on the results. Under what circumstances
does departure from ideal gas behavior increase?
Temperature Pressure
2008C
5 bar
3008C
5 bar
4008C
5 bar
5008C
5 bar
10008C
5 bar
2508C
0.1 bar
2508C
1 bar
2508C
5 bar
2508C
10 bar
2508C
25 bar
2-15. A refrigeration process includes a compressor, as
explained in detail in Chapter 5, because it is necessary to change the boiling point of the refrigerant,
which is done by controlling the pressure. Chapter 3 shows that the work required for compression is well approximated as equal to the change in
enthalpy. Use Appendix F to find the change in specific enthalpy for each of the scenarios A through C.
A. Freon 22 enters the compressor as saturated
vapor at P 5 0.5 bar and exits the compressor at
P 5 2 bar and T 5 208C.
B. Freon 22 enters the compressor as saturated
vapor at P 5 2 bar and exists the compressor at
P 5 8 bar and T 5 608C.
C. Freon 22 enters the compressor as saturated
vapor at P 5 5 bar and exists the compressor at
P 5 20 bar and T 5 80°C.
D. In these three compressors, the inlet and outlet
pressures varied considerably, but the “compression ratio” Pout/Pin was always 4. What do you notice
about the changes in enthalpy for the three cases?
79
2-17. Liquid water enters a steady state heat exchanger
at P 5 10 bar and T 5 80°C, and exits as saturated
water vapor at P 5 10 bar.
A. Using the steam tables, find the change in specific
enthalpy for this process.
B. Using the approximation that Cˆ p 5 4.19 J/g · K
for liquid water, find the change in enthalpy
when liquid water is heated from 80°C to its boiling point at P 5 10 bar.
C. How do the answers to parts A and B compare to
each other? What is the reason for the difference
between them?
2-18. One mole of ideal gas is confined in a piston-cylinder
device, which is 1 foot in diameter. The piston can
be assumed weightless and frictionless. The internal
and external pressures are both initially 1 atm. An
additional weight of 10 lbm is placed on top of the
piston, and the piston drops until the gas pressure
balances the force pushing the piston downward.
The temperature of the gas is maintained at a constant temperature of 80°F throughout the process.
A. What is the final pressure of the gas?
B. What is the final volume of the gas?
C. How much work was done on the gas during the
process?
2-19. Two moles of an ideal gas are confined in a pistoncylinder arrangement. Initially, the temperature is
300 K and the pressure is 1 bar. If the gas is compressed isothermally to 5 bar, how much work is
done on the gas?
N2 1 3H2 → 2 NH3
2-20. A gas is stored in an isochoric, refrigerated tank that
has V 5 5 m3. Initially, the gas inside the tank has
T 5 15°C and P 5 5 bar, while the ambient surroundings are at 25°C and atmospheric pressure.
The refrigeration system fails, and the gas inside the
tank gradually warms to 25°C.
A. Find the final pressure of the gas, assuming it is
an ideal gas.
B. Find the final pressure of the gas, assuming the
gas is described by the van der Waals equation of
state with a 5 8.0 3 105 cm6 · bar/mol2 and b 5
100 cm3/mol.
C. For the cases in parts A and B, how much work
was done by the gas on the surroundings?
This reaction is carried out at high pressures, most
often using an iron catalyst.
A. Use Equation 2.32 and C*P from Appendix D to
determine the change in molar enthalpy when
nitrogen is compressed from T 5 300 K and P 5 1
bar to T 5 700 K and P 5 200 bar.
B. Repeat part A for hydrogen.
C. What assumptions or approximations were made
in step B? Comment on how valid you think the
approximations are.
2-21. Biosphere II is an experimental structure that
was designed to be an isolated ecological space,
intended for conducting experiments on managed,
self-contained ecosystems. Biosphere II (so named
because the earth itself was regarded as the first
“Biosphere”) is covered by a rigid dome. One of
the engineering challenges in designing Biosphere
II stemmed from temperature fluctuations of the
air inside the dome. To prevent this from resulting in pressure fluctuations that could rupture the
2-16. The classic way to synthesize ammonia is through
the gas phase chemical reaction:
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80
Fundamentals of Chemical Engineering Thermodynamics
dome, flexible diaphragms called “lungs” were
built into the structure. These “lungs” expanded
and contracted so that changes in air volume could
be accommodated while pressure was maintained
constant.
For the purposes of this problem, assume that air is
an ideal gas, with heat capacity equal to a weighted
average of the ideal gas heat capacities of nitrogen
and oxygen: CP*,air 5 0.79CP*,N 1 0.21CP*,O .
2
2
A. Suppose the volume of air contained within the
Biosphere II was 125,000 m3, there were no lungs,
and the pressure of the air was 1 atmosphere
when the temperature was 70°F. What air pressure would occur at a temperature of 0°F?
B. Repeat part A, calculating the pressure resulting
if the temperature were 105°F rather than 0°F.
For parts C–E assume the Biosphere II has lungs,
and that the total volume of air inside the dome is
125,000 m3 when the lungs are fully collapsed.
C. If the lungs are designed to maintain a constant
pressure of 1 atm at all temperatures between
0 and 105°F, what volume of air must the lungs
hold when fully expanded?
D. If the temperature inside the dome increases
from 0°F to 105°F at a constant P 5 1 atm, what
change in internal energy does the air undergo?
E. If the temperature inside the dome increases
from 0°F to 105°F at a constant P 5 1 atm, and
the lungs are sized as calculated in part C, give
your best estimate of the work done by the lungs
on the surroundings.
Use the following data for Problems 2-22 through
2-26.
A compound has
A molecular weight of 50 lbm/lb-mol.
A normal melting point of 50°F.
A normal boiling point of 150°F.
A constant heat capacity in the solid phase of
Cˆ p 5 0.7 BTU/lbm·°R.
A constant heat capacity in the liquid phase of
Cˆ p 5 0.85 BTU/lbm·°R.
A constant density in the solid phase of
r 5 40 lbm/ft3.
A constant density in the liquid phase of
r 5 30 lbm/ft3.
An ideal gas heat capacity of Cˆ p* 5 1 2 T/1200 1
T 2 3 1026 BTU/lbm·°R with T representing the temperature in°R.
An enthalpy of fusion ΔĤfus 5 75 BTU/lbm at
atmospheric pressure.
An enthalpy of vaporization Δ Ĥvap 5 125 BTU/lbm
at atmospheric pressure.
2-22. Estimate the change in Û when this compound
melts at atmospheric pressure.
2-23. Estimate the change in Û when this compound boils
at atmospheric pressure.
2-24. When one lb-mol is heated from 25°F to 200°F at a
constant pressure of P 5 1 atm:
A. Give your best estimate of the change in enthalpy
B. Give your best estimate of the change in internal
energy
2-25. The compound is initially at 40°F. If the pressure is
constant at P 5 1 atm throughout the process, and
the specific enthalpy of the compound is increased
by 350 BTU/lbm, what is the final temperature?
2-26. The compound is initially at 175°F. If the pressure is
constant at P 5 1 atm throughout the process, and the
specific internal energy of the compound is decreased
by 150 BTU/lbm, what is the final temperature?
2-27. This question involves using the steam tables in
Appendix A, but answer questions A through C
before looking at the steam tables.
A. The density of liquid water at ambient conditions is about 1 g/cm3. Convert this into a specific
volume, expressed in m3/kg—the units used in
the steam tables.
B. You’re asked to look up the largest and smallest values of V̂ found anyplace in the steam
tables. Recall that Appendix A includes saturated steam tables, superheated steam tables,
and compressed liquid tables, and that the conditions in the tables range from 0.1–1000 bar
and 0–1000°C. Where will you look for the largest and smallest values?
C. Before looking up the largest and smallest values
of V̂, guess or estimate what they will be.
D. Look up the largest and smallest values of V̂ found
anyplace in the steam tables. How do they compare
to the guess/estimates you made in part C? Are they
located in the place you predicted in part B?
2-28 Model water using the van der Waals equation
of state with a 5 5.53 3 106 bar · cm6/mol2 and b 5
30.48 cm3/mol (The values of the van der Waals a and b
were determined using the method illustrated in Example 7-4).
A. Make a graph of P vs. V̂ for water at T 5 100°C.
Use the data from the steam tables.
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81
C H A P T E R 2 The Physical Properties of Pure Compounds
B. Using the van der Waals equation of state, plot a
graph of P versus V̂ for water, holding temperature
constant at T 5 100°C. Compare this to the data from
part A and comment on the quality of the predictions
produced by the van der Waals equation.
C. Repeat parts A and B for temperatures of 200°C,
300°C, 400°C, and 500°C. When you compare the
five temperatures to each other, what differences
in behavior do you notice, for both the real data
and the equation of state?
2-29. A compound has a molecular mass of 120 g/mol, and
the information in the table below is the only other
data available for a compound. Fill in all of the empty
cells with your best estimate of the value. Explain any
assumptions or approximations you make.
Phase
t
(°c)
P
(bar)
Û (kj/
kg)
A
Solid
0
5
242.6
B
Solid
25
5
0
C
Liquid
25
5
63.0
D
Liquid
60
5
E
Liquid
85
5
F
Vapor
85
5
977.2
G
Vapor
85
1
961.2
Ĥ (kj/
kg)
V̂
(m3/
kg)
8.0 3
1024
63.6
223.2
1014.2
2.7 g LO s s a r y O F s yM b O L s
A
area
a
parameter in van der Waals
EOS, accounting for intermolecular interactions
ĈV
CV
constant-volume heat
capacity (mass basis)
Q
Q
rate of heat addition
constant-volume heat
capacity (molar basis)
R
gas constant
T
temperature
U
internal energy
·
heat
b
parameter in van der
Waals EOS, accounting for
excluded volume
C*V
constant-volume heat
capacity for ideal gas
(molar basis)
Û
specific internal energy
C
number of chemical
compounds
F
degrees of freedom (in
Gibbs phase rule)
U
molar internal energy
V
volume
ĈP
constant-pressure heat
capacity (mass basis)
H
enthalpy
Ĥ
specific enthalpy
V̂
V
specific volume
molar volume
H
·
m
molar enthalpy
CP
constant-pressure heat
capacity (molar basis)
C *P
constant-pressure heat
capacity for ideal gas
(molar basis)
·
volumetric flow rate
mass flow rate
V
·
W
N
number of moles
p
number of phases
P
pressure
power
2.8 r e F e r e n c e s
Brown, R. L.; Stein, S. E. “Boiling Point Data,”
NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P. J. Linstrom and
W. G. Mallard, National Institute of Standards
and Technology, Gaithersburg MD, 20899, http://
webbook.nist.gov (retrieved July 6, 2012).
Cengel, Y. A.; Cimbala, J. M.; Turner, R. H. Fundamentals of Thermal-Fluid Sciences, 4th ed.,
McGraw-Hill, 2012.
Green, D. W. (editor) and Perry, R. H. (late editor),
Perry’s Chemical Engineers’ Handbook, 8th edition,
McGraw-Hill, 2008.
Laidler, K. J.; Meiser, J. H.; Sanctuary, B. C., Physical
Chemistry, Cengage Learning, Inc., 2003.
Machin, G. “System for Calibrating Thermometers,”
U.S. Patent #6,939,035, issued Sept. 6, 2005.
Olson, R. (editor), Biographical Encyclopedia of Scientists, Marshall Cavendish, Tarrytown, NY 1998.
Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P., The
Properties of Gases and Liquids. 5th edition,
McGraw-Hill, New York, 2001.
Reynolds, W. C., Thermodynamic Properties in SI,
Department of Mechanical Engineering, Stanford
University, 1979.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
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Material and Energy Balances
3
Learning Objectives
This chapter is intended to help you learn how to:
Define systems and processes that are well chosen for solution of a problem
Write material and energy balances that describe specific systems and processes
Recognize the implications of system and process descriptors such as adiabatic, steady
state, closed, or open, and how they are reflected in material and energy balance
equations
Identify physical properties that are necessary for solution of energy balance equations
Quantify relevant physical properties using simple thermodynamic models
Solve realistic engineering problems using material and energy balances
V
arious forms that energy can take and the fact that energy can be converted from
one form into another were discussed in Chapter 1. Indeed, to say energy “can be”
converted from one form into another isn’t going far enough; in the vast array of
engineering applications, energy conversion represents a major unifying theme. Almost all
of the products and systems you work with as a practicing engineer will use energy in some
form, and using energy efficiently is a major priority for modern engineers of all disciplines,
including chemical engineers. In this chapter, we examine quantitative analysis of energy
conversions.
3.1 MOtivatiOnaL exaMPLe: rockets
The first law of thermodynamics states that energy is conserved; it can be converted
from one form into another, but cannot be created or destroyed. In the first chapter
we examined the forms energy can take. Here we explore how to apply the first
law of thermodynamics to a broad range of problems. We’ve previously invoked the
principle of energy conservation to solve a problem. Example 1-6 examined an object in free fall. The first law of thermodynamics was relatively straightforward to
apply in this case, because energy in one form, potential energy, was completely converted into one (and only one) other form, kinetic energy. Many practical engineering systems and processes are more complex than this, with several forms of energy
relevant. Consider, for example, a rocket.
Conventional rockets operate through the combustion of liquid or solid fuel. As
the fuel burns, gases are produced. These gases are expelled out of the back of the
Robert Goddard, the
“Father of Modern
Rocketry,” is credited
with launching the
world’s first liquidfueled rocket in 1926.
It rose to a height of
41 feet and landed 184
feet from the launch
point. (Kluger, 1999)
83
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84
Fundamentals of Chemical Engineering Thermodynamics
The NASA probe
Voyager 1, launched
in 1977, was over
11.4 billion miles from
earth as of May 2013.
Updates on Voyager
1 have been available
at http://voyager.jpl.
nasa.gov/
One calorie is
classically defined as
the energy required to
raise the temperature
of one gram of
water by one degree
Celsius. What is now
called a “calorie” in a
nutritional context is
actually a kilocalorie by
the classical definition.
FOOd FOr
thOught
3-1
Is nitroglycerin,
the “most violent
explosive” known
at the time, the best
compound Bickerton
could have chosen to
make his point?
It isn’t actually possible
to transfer 100% of the
energy released when
the fuel is combusted
to the rocket in
the form of kinetic
energy. Example 3-13
illustrates some of the
reasons why.
Our intent is not
to criticize A. W.
Bickerton specifically.
Many people were
skeptical of the concept
of rocketry and critical
of Robert Goddard.
In 1920, a New York
Times editorial said
that Goddard “seems
to lack the knowledge
ladled out daily in high
schools.” The Times
printed a retraction in
1969. (Kluger, 1999)
rocket through nozzles, at extremely high velocity. As the gases move downward,
the rocket is propelled upward through conservation of momentum. In this process,
internal energy, kinetic energy and potential energy are all important; and there are
two distinct velocities (that of the rocket and that of the exhaust) to consider in
quantifying kinetic energy. Accounting for the energy conversion is much more complicated than in the simple case of a falling object, and it would be very easy to make
an error. One such error is illustrated below.
In the early days of rocketry, many were doubtful that the technology could be
used to reach outer space. Indeed, here is a “proof,” attributed to A. W. Bickerton
(Clarke, 1962), in which the first law of thermodynamics was used to demonstrate
that space flight is impossible:
1. The velocity needed to escape the Earth’s gravity—the escape velocity—is
7 miles per second.
2. The kinetic energy of a gram at this velocity is 15,180 calories.
3. The energy of the most violent explosive—nitroglycerin—is less than 1500 calories
per gram. (The calorie was a commonly used unit for energy in the 1920s; a calorie
is equal to 4.184 J.)
The argument is that even if the fuel had nothing to carry, it has less than onetenth the energy required to escape the Earth’s gravity, and that space flight is, therefore, impossible.
This proof is clearly an attempt to apply the principle of conservation of energy,
but it must be mistaken in some respect, since we now know space flight is indeed
possible. Before you read further, can you spot the error in the “proof”? The math is
not the problem-the numbers are all accurate.
The crucial point is: the rocket fuel doesn’t have to get into outer space. If you’ve
ever seen a rocket launch, you know that a great deal of fuel is burned and released
very close to the ground. It is the payload (the rocket itself, equipment, astronauts,
etc.) that needs to get into space. In Bickerton’s time, kerosene was widely used as a
fuel for lighting and heating. Ten grams of a kerosene/oxygen mixture releases over
20,000 calories when burned. If 100% of this energy is transferred to the payload
in the form of kinetic energy, then ten grams of fuel is more than enough to propel
one gram of payload into outer space. While modern rockets use better fuels than
kerosene, we have now refuted the above “proof” using information that was available in the 1920s.
How can we avoid making errors such as the one illustrated above? One principle that is helpful is that of a system, introduced in Section 1.3. One cannot answer
questions or draw conclusions about a system or process without a clear understanding of what that system or process is. In this case, “the rocket and all of its contents”
would be a logical definition of the system. Next, we can account for all the energy
present in the system, which is illustrated in Figure 3-1. At the start of the process, the
system is at rest (kinetic energy 5 0) on the ground (potential energy 5 0), but contains both payload and rocket fuel (the fuel has a large internal energy). At a later
point, the system contains no fuel, but has significant potential and kinetic energy
that can be quantified if height and velocity are known.
Figure 3-1 and the above discussion might help us rationalize why Bickerton’s
“proof” is wrong, but it leaves significant questions unanswered. How much energy is
actually released when the fuel burns? What form does this energy take, specifically?
What about the combustion products-presumably primarily carbon dioxide and
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C H A P T E R 3 Material and Energy Balances
85
Final State
Initial State
v50
h50
Contains
payload
and fuel
v 5 large
h 5 large
Contains
payload,
no fuel
Figure 3‑1 In a rocket launch, the internal energy of the fuel is transferred to the rocket in
the forms of kinetic energy (Mv2y2) and potential energy (Mgh).
water? These gases do not simply disappear; they have stored energy (internal,
potential and kinetic) that they take with them when they leave the system. How
does this phenomenon influence the velocity and height attained by the rocket?
What about air resistance? Rockets move quite quickly, and the faster an object
moves, the more air resistance it encounters. Since the air represents a force opposing the rocket’s motion, this falls into the category of work.
There is no single, simple answer to the questions raised in the last paragraph.
In analyzing a system that involves energy conversion, one must consider all of the
forms in which energy can be stored or transferred, and for each, either determine
it is negligible for the case at hand or else account for it quantitatively. Section 3-3
presents a mathematical expression of the first law of thermodynamics, which is
called the generalized energy balance, that makes such an analysis easier.
The focus of this chapter is energy balances, but energy balances are often applied in combination with material balances. For example, in analyzing a rocket,
one cannot quantify the stored energy in the exhaust gases without knowing how
much gas is actually released. A material balance allows one to relate the amounts
of carbon dioxide and water produced to the amount of fuel burned. Consequently,
this chapter examines material and energy balances, applied to physical systems
both individually and in combination with each other. We begin with material
balances.
Remember, the
fuels we burn are
predominantly
hydrocarbons, and
carbon dioxide and
water are the products
when a hydrocarbon is
burned to completion.
The space shuttle
Endeavour, which was
retired in 2011, had a
mass of ~172,000 lbs
when empty and
usually carried
about 53,000 lbs of
additional payload.
Over 3.8 million
pounds of fuel were
used to launch the
orbiter. (http://www.
nasa.gov, accessed
October 2013.)
3.2 Material balances
This section discusses how to apply and solve material balances. Section 3.2.2 contains three worked examples, but first we develop the material balance equation in a
form that is applicable to any system.
Material balances are
also frequently called
“mass balances.”
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86
Fundamentals of Chemical Engineering Thermodynamics
FOOd FOr
thOught
3-2
What would the
material balance
look like for the
Motivational Example
process illustrated in
Figure 3-1?
3.2.1 Mathematical Formulation of the Material balance
A balance equation can be envisioned as
Accumulation 5 In 2 Out 1 Generation 2 Consumption
We are modeling total mass as a conserved quantity. Nuclear processes exist where
mass and energy are not conserved, but these are not considered in this book. The
principle of conservation of mass is reflected mathematically in that there is no “generation” or “consumption” term. When our material balance represents the total
mass of the system, it simplifies to
Accumulation 5 In 2 Out
FOOd FOr
thOught
3-3
What if a chemical
reaction occurred, and
water were converted
into hydrogen and
oxygen? How would
this be reflected in the
material balance?
For example, consider the water in a swimming pool as a system. We can imagine
events that affect the amount of water in the pool: water can evaporate, rain can fall,
one could fill the pool with a garden hose, etc. If water is added (in) more quickly
than it is removed (out), the amount of water in the pool will increase, and this is
reflected mathematically by a positive value for the ‘accumulation’ term. (A swimming pool is examined quantitatively in Problem 3-8.)
Using the nomenclature of this book, the balance equation for total mass is
j5J
k5K
o
·
dM
· 2
m
m
5
(3.1)
k ,out
j,in
dt
k51
j51
· is the mass flow rate of a stream entering
where M is the total mass of the system, m
o
or leaving the system, t is time. J is the total number of entering streams, K is the total
number of exiting streams, and j and k are subscripts used to distinguish entering and
exiting streams from each other.
FOOd FOr
thOught
3-4
A swimming pool
normally has a
circulating filtration
system that removes
water, filters it and
returns it to the pool.
How would this be
included in a material
balance?
Since M represents total mass of the system, the left-hand side dMydt represents “accumulation;” the rate at which the total mass is changing. The right-hand side repre· represents a mass
sents material flows entering or leaving the system. The symbol m
flow rate, and has the dimension mass/time; common units include kg/s or lbm/min.
The summations ( o ) account for the fact that there may be several different sources
of mass entering or leaving the system. For example, Figure 3-2 shows a swimming
min,rain
min,hose
Figure 3‑2 Mass is added to a swimming pool from two different sources, or
“streams”: rain and a garden hose.
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C H A P T E R 3 Material and Energy Balances
87
pool that is being filled with a garden hose while rain is also falling. Consequently,
if the pool were the system, there would be two separate “in” terms, which might be
· and m
· , or m
·
·
called m
and m
. Distinct sources of mass entering or leaving
1,in
2,in
in,hose
in,rain
the system are typically called “streams.”
The next section illustrates the use of this equation through examples.
3.2.2 examples of Material balances
The material balance equation is applied using three examples.
Example 3-1 examines steam in a piston-cylinder device; an unsteady-state process carried out in a closed system.
Example 3-2 examines a bathroom sink with the water left on. We use an opensystem material balance to determine when the sink will overflow.
Example 3-3 examines the growth of an oak tree, and requires three simultaneous material balances to solve.
cOMPressiOn OF steaM
examPle 3‑1
A vessel initially contains 5.0 m3 of superheated steam at T 5 300°C and P 5 1 bar. The
vessel is sealed but is equipped with a piston-cylinder arrangement so that the volume
can be changed (Figure 3-3). If the steam is compressed isothermally to P 5 5 bar, what
will be the new total volume?
Initial State
V1 5 5.0 m3
T1 5 3008C
P1 5 1 bar
Final State
V2 5 ?
T2 5 3008C
P2 5 5 bar
Figure 3‑3 Initial and final states of the steam in the vessel
described in Example 3-1.
SOLUTION:
Step 1 Define the system
We’re trying to predict the final volume of steam, so define the system as the steam.
Step 2 Gather data
We saw in Chapter 2 that, if two intensive properties of a pure compound in a single
phase are known, all others will have unique values. Here T and P are known for both
In a steady-state
process, ALL
parameters of the
system are constant
with respect to time.
Here, temperature
is constant, but
pressure changes with
time, so there is no
steady state.
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88
Fundamentals of Chemical Engineering Thermodynamics
pitFaLL
preventiOn
If you are careless
with units, you could
confuse the specific
volume (m3/kg) with
the total volume (m3).
the initial and final states. Thus, specific volumes can be obtained from the superheated
steam tables in Appendix A-3:
V̂ 5 0.5226
V̂ 5 2.639
m3
at P 5 5 bar and T 5 300°C
kg
m3
at P 5 1 bar and T 5 300°C
kg
Step 3 Material Balance
We know the specific volume at the end of the process, but we also need to know the total
mass in order to find the total volume. Thus, doing a material balance is a logical step:
k5K
j5J
·
dM
· 2
m
m
5
k,out
j,in
dt
k51
j51
o
o
Equation 3.3 is a
material balance
equation that could be
used to describe any
closed system.
(3.2)
Here the vessel is described as “sealed;” no mass enters or leaves. Thus, the mass balance
simplifies to:
dM
50
dt
(3.3)
Consequently, M is constant; the initial and final masses of the system are equal.
Step 4 Find initial value of M
At the initial state, total volume and specific volume are known, so the total mass can be
determined:
FOOd FOr
thOught
3-5
Could you have
solved this problem
using the ideal gas
law? How would
the answer have
compared?
Equation 3.34 in
Section 3.3 is a
generalized energy
balance, analogous to
the material balance
in Equation 3.1.
examPle 3‑2
M5
V
V̂
5
5.0 m3
5 1.89 kg
m3
2.639
kg
(3.4)
Step 5 Calculate final volume
We proved in step 3 that mass is constant; thus, the unknown final volume is
1
V 5 MV̂ 5 s1.89 kgd 0.5226
2
m3
5 0.99 m3
kg
(3.5)
In Example 3-1, going through the process of formally writing out the mass balance
equation probably seems unnecessary. The insight that “mass is constant,” while necessary to solve the problem, can be reached without writing a differential equation.
One could, therefore, have skipped step 3 and probably still arrived at a correct
answer. However, as we move forward and examine more and more complex examples, you will inevitably encounter many examples where the needed insights are
not intuitively obvious and there are subtle points that could easily be overlooked.
The value of generalized balance equations, like Equations 3.1 and 3.34, is that they
are always valid; applying them helps ensure you don’t overlook anything and they
give you a starting point when the solution strategy isn’t obvious.
OverFLOWing bathrOOM sinK
A 4-L sink is initially half-full of water. The faucet is turned on, and 1 kg/min of water
begins pouring into the sink. At the same moment that the faucet is turned on, the drain
is opened, and 10 cm3/sec of water flows out of the sink (Figure 3-4). How long after the
faucet is turned on will the sink overflow?
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C H A P T E R 3 Material and Energy Balances
min 5 1 kg/min
min 5 1 kg/min
V52L
The water in the sink
is an example of an
open system, and is
not at steady state,
because the mass of
the system is changing.
V54L
v 5 10 cm3/sec
v 5 10 cm3/sec
Figure 3‑4 Initial and final states of the bathroom sink
described in Example 3-2.
SOLUTION:
Step 1 Define the system and process
The sink will overflow when the volume of water exceeds 4 L. Consequently, we define
the water in the sink as the system, the start of the process as the moment the faucet
is turned on, and the end of the process as the moment at which the volume of water
equals 4 L.
Step 2 Material balance
Start with the general mass balance in Equation 3.1:
j5J
k5K
dM
·
· 2
5
m
m
k,out
j,in
dt
k51
j51
o
o
Since there is only one stream entering the system, and one stream leaving, this equation
simplifies to
dM
·
· 2m
5m
out
in
dt
(3.6)
Step 3 An assumption
Some of the given information is on a mass basis, and some on a volume basis. We can’t
solve the mass balance equation until everything is expressed on a mass basis. Since no
information about temperature and pressure are given, we assume the density of water
is the standard 1 g/cm3, or 1 kg/L.
pitFaLL
preventiOn
Volume is not a
conserved quantity;
we cannot in general
write “volume
balances.”
Step 4 Unit conversions
The initial volume V0 of the water is 2 L, so its initial mass M0 is
M0 5 V0 r 5 s2 Ld
1 L 2 1 kg 2 5 2000 g
1 kg
1000 g
(3.7)
Analogously we can determine that the sink will be full when M 5 4000 g.
The rates at which water leaves and enters the system are
1
· 5 10 cm
m
out
s
1
· 5 1 kg
m
in
min
3
2 1cm 2 5 10 s
1g
g
(3.8)
3
2 1160mins 2 5 16.67 s
g
89
(3.9)
FOOd FOr
thOught
3-6
How would the
problem be different
· turned out
if m
out
to be bigger when
· and m
· were
m
in
out
converted into the
same units?
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90
Fundamentals of Chemical Engineering Thermodynamics
Step 5 Solve material balance
· are both constant, the material balance is readily solved by separation
· and m
Since m
out
in
of variables:
dM
·
· 2m
5m
out
in
dt
#
M5M
dM 5
M5M0
(3.10)
# sm· 2 m· d dt
t5t
in
t50
out
· 2m
· dt
M 2 M0 5 sm
in
out
Substituting in known values:
1
M 2 2000 g 5 16.67 2 10
2
g
t
s
(3.11)
When M 5 4000 g, t 5 300 sec or t 5 5 min.
The next example involves using multiple material balances in conjunction. It also
illustrates a common feature of thermodynamics problems in that it describes a process (the growth of a tree) in which the final state (the way the tree looks right now)
and the initial state (when there was no tree at all) are well understood, but the duration of the process is unknown.
examPle 3‑3
This problem ignores
the leaves, branches
and roots of the tree,
which are also made
of hydrocarbons, so
the real CO2 required
to grow a 30-foot-tall
tree is even larger
than this problem
illustrates.
grOW th OF a tree
An oak tree is 30 feet tall and 2 feet in diameter (Figure 3-5). How many pounds of CO2
and H2O were consumed to form the tree’s trunk? How many pounds of O2 gas were
released? Assume the density of oak wood is 45 lbm/ft3 and that the trunk is pure cellulose
(C6H10O5) and is perfectly cylindrical in shape.
r 5 45 lb/ft3
Cellulose 5 C6H10O5
30 ft
2 ft
Figure 3‑5 The tree described in Example 3-3.
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C H A P T E R 3 Material and Energy Balances
91
SOLUTION:
Step 1 Define the system and process
Define the tree trunk as the system, and define the process to encompass the entire
growth of the tree.
Step 2 Write overall material balance
The tree takes in water and carbon dioxide and generates oxygen. Thus, the overall mass
balance is
dM
·
·
·
2m
1m
5m
O
HO
CO
dt
2
(3.12)
2
2
Initially there is no tree, so M 5 0, and the final mass can be determined from the given
information, but this still leaves three unknowns on the right hand side and only one
equation.
Step 3 Recognize additional material balances
We can write a balance equation analogous to Equation 3.12 not only for total mass, but
for the quantity of any element. Instead of M for mass we will use N for moles.
j5J
k5K
dN
5
n· k,out
n· j,in 2
dt
k51
j51
o
o
Step 4 Apply simplifying assumptions
We assume that ALL of the carbon, hydrogen, and oxygen for the tree trunk came from
CO2 and H2O; no other compounds. This may not be literally true, but it’s a logical approximation and one that is implied by the wording of the question. The balance equations are
dt
5 2n· H O
(3.14)
2
dNC
5 n· CO
(3.15)
dt
In which NH and NC represent the numbers of moles of hydrogen and carbon atoms in
the tree, and n· H O and n· CO are the molar flow rates at which water and carbon dioxide are
added to the system.
2
2
2
Step 5 Examine the degrees of freedom
We’re asked to solve for three unknowns: water in, carbon dioxide in, and oxygen out.
Conceptually, we need three independent equations to solve for three unknowns. We
now have three equations, but they contain additional variables that must be computed:
M, NH, NC, and t.
Step 6 Calculate quantities describing final state of system
The volume of a cylinder is r2 h, so the FINAL mass (M) of the tree is
1 ft 2 s3.1416d s1 ftd s30 ftd 5 4241 lb
M 5 rV 5 rr2 h 5 45
lbm
2
3
m
(3.16)
We’re told cellulose has the empirical structure C6H10O5, which means a molecular mass
of 162. Thus,
Ncellulose 5
This book does not
consider nuclear
processes, in which
atoms are converted
into other atoms. Thus,
we consider “total
mass” and “number of
C atoms” as conserved
quantities, but we
should think of these
as useful models, not
as absolute “truths.”
(3.13)
Here we will write material balances for carbon and hydrogen atoms.
dNH
The tree trunk is an
example of an open
system; mass is added
as the tree grows.
4241lbm
M
5
5 26.18 lb-mol
MW
lb
162
lb-mol
(3.17)
FOOd FOr
thOught
3-7
Why are carbon and
hydrogen chosen here,
but not oxygen?
Note the stoichiometric
coefficient of 2 in
Equation 3.14, which
represents the fact
that when one mole of
water is added to the
system, two moles of
hydrogen atoms are
added.
pitFaLL
preventiOn
Keep in mind, this
is an unsteady-state
problem; M, NH , NC,
and t each have an
initial value and a
final value. There is
no tree when t 5 0,
so in this case, M, NH ,
and NC all have initial
values of zero. But
many problems have
variables with nonzero initial values.
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92
Fundamentals of Chemical Engineering Thermodynamics
Recall that “the
process” as we defined
it began when there
was no tree trunk
at all.
FOOd FOr
thOught
3-8
If you plotted the rate
of tree growth with
respect to time, what
would you expect it to
look like?
Thus, at the END of the tree’s growth, the moles of carbon and hydrogen are
NC 5 6Ncellulose 5 6 s26.18 lb-mold 5 157.1 lb-mol
(3.18)
NH 5 10Ncellulose 5 10 s26.18 lb-mold 5 261.8 lb-mol
(3.19)
Thus, we know the exact composition of the tree at the end of the process, and at the
beginning M, NC, and NH are all zero. But how do we handle the time-dependence of the
material balances?
Step 6 Integration of differential equations
We have no information regarding how long the growth took or the rate at which water
and carbon dioxide were added. In such a case, time can be eliminated from the material balance equations by integrating with respect to dt. Multiplying both sides of Equation 3.12 by dt gives
·
·
· d dt
1m
2m
dM 5 sm
CO
HO
O
2
This definition of
m can be applied
even if (as in this
example) we have
no quantitative
information on what
· are.
tfinal and m
pitFaLL
preventiOn
It’s easy to confuse
values of mass and
mole in a complex
problem. In our
nomenclature, capital
M or N refers to
mass or moles of the
system, and lowercase
m or n refers to mass
or moles entering or
leaving the system.
2
(3.20)
2
· ) is not necessarily constant with respect to time.
The rate at which mass is added (m
·
However, if m is integrated with respect to time from the beginning of the process to the
end, the result is the total mass added (or removed). This quantity m is
m5
#
t5tfinal
· dt
m
(3.21)
t50
If we apply this definition to the CO2 and H2O added, and the O2 removed, Equation 3.20
integrates to
Mfinal 2 Minitial 5 mCO 1 mH O 2 mO
2
2
(3.22)
2
Analogous to Equation 3.21, we define n as the total number of moles added (or removed) to the system over the entire process:
n5
#
t5t final
n· dt
(3.23)
t50
And integrating Equations 3.14 and 3.15 with respect to dt, just as we did with Equation 3.22, gives
NH,final 2 NH,initial 5 2nH O
(3.24)
NC,final 2 NC,initial 5 nCO
(3.25)
2
2
Step 7 Plugging in known values
Inserting values from step 5 into the integrated equations from step 6 gives
nH O 5
2
261.8 lb-mol
5 130.9 lb-mol
2
(3.26)
nCO 5 157.1 lb-mol
(3.27)
2
Converting to mass gives
The growth of this
tree’s trunk took
approximately three
and a half tons of
carbon dioxide out of
the atmosphere.
1 lb-mol2 5 2356 lb
mH O 5 s130.9 lb-mold 18
2
1 lb-mol2 5 6912 lb
mCO 5 s157.1 lb-mold 44
2
lbm
m
(3.28)
m
(3.29)
lbm
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C H A P T E R 3 Material and Energy Balances
93
Substituting these values into Equation 3.22 gives
4241 lbm 2 0 5 2356 lbm 1 6912 lbm 2 mO
(3.30)
2
mO 5 5027 lbm
2
Example 3-3 illustrates some ideas that will be recurring themes in the study of
thermodynamics. First, while the most general form of the material balance is the
time-dependent Equation 3.31:
dM
5
dt
j5J
k5K
o m· 2 o m·
j,in
k,out
k51
j51
There are many examples, such as the growth of the tree, in which no information is
available about the duration or rate of the process. Furthermore, no such information is needed: We know the initial state (no tree) and the final state (the current
tree), and we can answer the given questions without knowing how old the tree is or
what the tree looked like at any particular intermediate time.
For such examples, where no information on time is available or needed, we can use a
time-independent mass balance:
j5J
Mfinal 2 Minitial 5
k5K
om 2 om
j,in
j51
Equation 3.31 can be
derived from Equation
3-1 by multiplying
through by dt and
integrating, as in step 6
of Example 3-3.
k,out
(3.31)
k51
where m represents the total mass added from, or removed by, one stream throughout the entire process, and Minitial and Mfinal represent the masses of the system at the
beginning and end of the process.
FOOd FOr
thOught
3-9
Another principle illustrated in Examples 3.1 through 3.3 is that we can write material balances on total mass, and also on the amount of any element. We can also write
material balances for compounds (e.g., a “water balance” rather than a “hydrogen
balance”), but the amount of a compound is not always conserved quantity. A material balance for water, or any other compound, must include a term accounting for
generation and/or consumption, at least for cases where chemical reactions occur.
Chapters 1 through 8 of this book focus on applications in which the system
contains only a pure compound, so in these chapters we won’t typically have need
of material balance equations beyond the “total mass” balance. Mole balances on
specific chemical species are of particular importance in Chapters 14 and 15, which
cover chemical reactions.
To model the rocket
process illustrated in
Figure 3-1, would you
use Equation 3.1 or
Equation 3.31?
3.3 Mathematical expression of the First Law of thermodynamics
As indicated in Section 1.4, the first law of thermodynamics states that energy can
be converted from one form into another, but cannot be created or destroyed. This
fundamental principle is applicable to an enormous range of engineering problems. “How much work (or heat) would it take to do this job?” is one example of a
straightforward, practical type of engineering question that can be answered using
the principle of conservation of energy.
In Chapter 1, the first law was invoked in the solution of Example 1.6, which examined an object in free fall. In that example, it was assumed that as the object fell,
potential energy was converted into kinetic energy, and no other forms of energy
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94
Fundamentals of Chemical Engineering Thermodynamics
Energy balances are
frequently applied
in combination with
other equations: mass
balances, equations
of state, etc. Counting
the number of
degrees of freedom,
or independent
unknowns, in a
problem allows you to
determine how many
relevant equations
must be identified or
additional variables
specified.
were significant. An object in free fall is a fairly simple physical system. In more
complex systems, one could easily overlook one or more contributions to the energy
of the system. Consequently, in this section, we develop a comprehensive, formal
mathematical statement of the first law of thermodynamics, which we will call the
generalized energy balance equation. Further, we recommend a methodical strategy
for applying the energy balance:
1. Identify a system that has clearly recognizable boundaries.
2. Go through each form of energy represented in the generalized energy balance
(e.g., work, heat, potential energy, kinetic energy) and carefully consider whether
it is negligible or significant for the chosen system. Simplify the equation by setting the negligible terms to zero with justification.
3. For those equation terms which are, or may be, significant, determine a strategy
for relating them to the known information.
Sections 3.4 and 3.5 contain several worked examples in which this problem-solving
strategy is applied. First, we develop the energy balance equation itself.
As stated in Section 3.2, any balance equation can be described in words as
Accumulation 5 In 2 Out 1 Generation 2 Consumption
The first law of thermodynamics states that energy is a conserved quantity; it can be
converted from one form into another, can be transferred from location to location,
but cannot be created or destroyed. Thus, for energy, the generation and consumption terms do not exist:
Accumulation 5 In 2 Out
Chapter 1 describes three forms in which energy can be stored by matter: internal
energy, kinetic energy, and potential energy. Thus, the accumulation term must reflect
changes in any or all of these.
Accumulation 5
5
d
sinternal 1 kinetic 1 potentiald
dt
5 1
d
v2
M Û 1 1 gh
dt
2
(3.32)
26
The accumulation term reflects changes in the total energy contained within the system. If matter is added to, or removed from, the system, the matter entering or leaving also has stored energy. Thus, the “in” and “out” terms must reflect the internal,
kinetic, and potential energy of matter entering or leaving. These terms must also
reflect the flow work done on the system as material enters or leaves the system; this
phenomenon was explained in Section 1.4.3.
In addition, energy can be transferred to or from the system in the forms of heat
and mechanical work. We use the symbol WEC for work of expansion or contraction,
WS for shaft work, and Q for heat, and these terms have positive signs if energy is
added to the system and negative signs if energy is removed from the system.
The complete energy balance is as follows:
The PV̂ term in the
generalized energy
balance represents
flow work.
5 1
d
v2
M Û 1 1 gh
dt
2
vj2
1 gh 26
(3.33)
26 5 o 5 1
2
v
·
2 o 5m
1Û 1 P V̂ 1 2 1 gh 26 1 W· 1 W· 1 Q·
j5J
j51
·
m
Ûj 1 PjV̂j 1
j,in
j
2
k
k5K
k51
k,out
k
k
k
k
S
EC
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C H A P T E R 3 Material and Energy Balances
Note that a Û 1 PV̂ term appears for all streams entering or leaving the system.
Recall that the property enthalpy was defined in Section 2.3.2 as
H 5 U 1 PV
This can be divided through by either moles (HyN 5 H ) or mass (HyM 5 Ĥ) to give
H 5 U 1 PV
Ĥ 5 Û 1 PV̂
or
Introducing the specific enthalpy into the right-hand side of the equation causes the
energy balance to simplify to the following form, which is the one that is conventionally used.
The generalized, time-dependent energy balance is written as
51
(3.34)
26 5 o 5m· 1Ĥ 1 2 1 gh 26
v
·
2 o 5m
1Ĥ 1 2 1 gh 26 1 W· 1 W· 1 Q·
v 2j
j5J
d
v2
M Û 1 1 gh
dt
2
j ,in
j
j
j51
k5K
2
k
k ,out
k
k
S
EC
k51
where
FOOd FOr
thOught
3-10
What would the
generalized energy
balance look like
for the rocket in
Figure 3-1?
t is time.
M is the total mass of the system.
Û is the specific internal energy of the system.
v is the velocity of the system.
h is the height of the system.
g is the acceleration due to gravity, equal to 9.81 m/s2 on Earth.
· and m
·
m
are the mass flow rates of individual streams entering and leaving
j,in
k,out
the system, respectively, and the summations are carried out over all such streams.
Ĥj and Ĥk are the specific enthalpies of streams entering and leaving the system.
vj and vk are the velocities of streams entering and leaving the system.
hj and hk are the heights at which streams enter and leave the system.
·
WEC is the rate at which work is added to the system through expansion or
contraction of the system.
·
WS is the rate at which shaft work is added to the system.
·
Q is the rate at which heat is added to the system.
Example 3-3 demonstrated that in some applications it is impractical and unnecessary to quantify the relationship between process variables and time. Through that
example, an integrated, “time-independent” form of the mass balance equation was
developed.
The analogous time-independent form of the energy balance is
51
D M Û 1
26 o 5 1
v2
1 gh
2
j5J
5
mj,in Ĥj 1
v 2j
j51
o 5m
k5K
2
k51
1
2
1 ghj
Ĥk 1
k,out
v 2k
2
26
(3.35)
26 1 W 1 W 1 Q
1 ghk
EC
S
(Continued)
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95
96
Fundamentals of Chemical Engineering Thermodynamics
where
WEC is the total expansion/contraction work added throughout the process.
WS is the total shaft work added throughout the process.
Q is the total heat added throughout the process.
mj,in and mk,out are individual quantities of mass added to and removed from the
process, respectively, and the summations are carried out over all such quantities.
Equations 3.34 and 3.35 are both completely general, and therefore valid for any
system. Which one is more convenient to use for a particular problem depends on
whether or not time shows up explicitly in either the known information (e.g., “Heat
is added at a rate of 10 kJ/min”) or in the questions to be answered (e.g., “How long
will it take for the water to reach boiling temperature?”)
The next two sections are devoted to applying the energy balance—sometimes
in combination with the mass balance—to a variety of examples. These examples
are also used to illustrate some of the key engineering unit operations that are of
interest throughout the book.
3.4 applications of the generalized energy balance equation
The first example shows a nozzle, a device that is used to accelerate a flowing fluid to
high velocity. A nozzle has a tapered neck, as shown in Figure 3-6, and the decreasing
diameter creates a large pressure drop and forces a high velocity. Typically, no energy
is transferred to the fluid from external sources.
exaMPLe 3-4
The term “adiabatic”
was first defined in
Section 1.3 along with
several other essential
definitions used to describe thermodynamic
systems and processes.
FOOd FOr
thOught
3-11
This example is in the
energy balance section,
so of course there’s an
energy balance. But
suppose your boss
asked you to calculate
the velocity coming
out of a nozzle. How
would you recognize
this as an “energy
balance” problem?
nOzzLe
If steam flows through an adiabatic nozzle at steady state, entering the nozzle with a
pressure of 5 bar and a temperature of 4008C and exiting at 1 bar and 3508C (Figure 3-6),
what is its exiting velocity?
Tout 5 3508C
Pout 5 1 bar
Tin 5 4008C
Pin 5 5 bar
Figure 3‑6 An adiabatic nozzle at steady state
described in Example 3-4.
SOLUTION:
Step 1 Define the system
Defining the nozzle as the system allows us to make use of the described properties of
the nozzle (e.g., it’s adiabatic, it operates at steady state).
Step 2 Apply and simplify the energy balance
At steady state, the accumulation term is 0, and the time-independent energy balance
simplifies to
o5
j5J
05
j51
1
mj,in Ĥj 1
v 2j
2
26 2 o 5m 1Ĥ 1 2 1 gh 26 1 W 1 W 1 Q (3.36)
v 2k
k5K
1 ghj
k,out
k
k
EC
S
k51
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C H A P T E R 3 Material and Energy Balances
Examining the right-hand side of the equation, note:
There is only one stream entering (min) and only one stream leaving (mout).
Because this is a steady-state process with only one entering stream and only one
exiting stream, the material balance (Equation 3.31) simplifies to min 5 mout.
Q 5 0 since the problem statement says the nozzle is adiabatic.
WEC 5 0 because the boundaries of the system are not moving.
97
FOOd FOr
thOught
3-12
Would it have been
ok to use the timedependent energy
balance instead?
WS 5 0 because the system contains no moving parts or any mechanism for shaft work.
Thus, the energy balance further simplifies to:
1
0 5 min Ĥin 1
v2in
2
2
1
1 ghin 2 mout Ĥout 1
v 2out
2
1 ghout
2
(3.37)
Because min5 mout, we can simply divide both sides by this mass:
1
0 5 Ĥin 1
v 2in
2
2 1
1 ghin 2 Ĥout 1
v 2out
2
1 ghout
2
(3.38)
According to Figure 3-6 the nozzle is oriented horizontally, so there is no difference in
height between the entering and leaving streams. If hin5hout, the potential energy terms
cancel each other out.
1
0 5 Ĥin 1
v 2in
2
2 1
2 Ĥout 1
v 2out
2
2
1
v 2out
2
2
(3.40)
which can be rearranged as
v 2out
2
If the nozzle were
oriented vertically,
would potential
energy be significant?
(3.39)
Step 3 Simplifying assumption
The exiting velocity (vout) is what we are trying to determine. There is no information in
the problem statement that would enable us to determine the inlet velocity (vin), so we
will assume it is small enough that the entering kinetic energy is negligible. Thus,
0 5 sĤin d 2 Ĥout 1
FOOd FOr
thOught
3-13
5 Ĥin 2 Ĥout
Recall from
Section 1.2.3 that a
nozzle is an integral
component of a
turbine. This example
shows internal energy
of steam being
converted to kinetic
energy.
(3.41)
This is a typical energy balance for nozzles. The result is somewhat intuitive in that the
purpose of the nozzle is to accelerate the fluid to higher velocity, and Equation 3.41
shows that a portion of the entering fluid’s enthalpy is converted into kinetic energy.
Step 4 Plug in numerical values
The temperatures and pressures of the entering and leaving streams are known so the
enthalpies can be found from the superheated steam tables (Appendix A-3) as
Ĥin (T 5 4008C, P 5 5 bar) 5 3272.3 kJ/kg
Ĥout (T 5 3508C, P 5 1 bar) 5 3175.8 kJ/kg
Inserting these into Equation 3.41 and performing the necessary unit conversions gives
v 2out
2
1
5 3272.3 2 3175.8
kJ
kg
21
1000
J
kJ
vout 5 439.5
21
m
s
1
N?m
J
2
1 2
kg ? m
s2
1
N
(3.42)
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98
Fundamentals of Chemical Engineering Thermodynamics
439 meters per second
is 977 miles per hour.
exaMPLe 3-5
In this process, the
exiting steam is
superheated; it is at a
temperature above its
boiling temperature.
The speed of sound moving through air at ambient temperature and pressure is
about 343 m/s, so the velocity of 439.5 m/s obtained in Example 3-4 is a high velocity,
but not physically unrealistic. The next example considers velocities that are more
commonly found in chemical process equipment.
steady-state bOiLer
A boiler operates at steady state. The entering water is saturated liquid at P 5 5 bar and
has a flow rate of 10,000 kg/hr. The exiting steam is also at P 5 5 bar and has T 5 4008C.
The pipe entering the boiler is 8 cm in diameter, and the pipe leaving the boiler is 30 cm
in diameter (Figure 3-7). What is the rate at which heat is added in the boiler?
Boiler
Pin 5 5 bar
min 5 10,000 kg/hr
Saturated liquid
Pout 5 5 bar
Tout 5 4008C
D 5 30 cm
D 5 8 cm
Figure 3‑7 A boiler operating at steady state, as described in Figure 3-5.
FOOd FOr
thOught
3-14
This boiler is
producing steam at
the same conditions
that entered the
nozzle in Example 3-4.
Combined, the
boiler and the nozzle
effectively convert
heat into kinetic
energy. How is this
like what goes on in
a rocket? How is it
different?
The boiler is an
example of an open
system.
SOLUTION:
Step 1 Define the system
Define the contents of the boiler as the system.
Step 2 Apply and simplify the energy balance
We use the time-dependent form of the energy balance, because we are trying to determine the rate at which heat is added. Because the system operates at steady state, the accumulation term of the energy balance is zero. We can neglect all work terms, because the
boundaries of the system do not move, and there isn’t normally any shaft work in a boiler
(or any other conventional heat exchanger). Finally, there is only one stream entering the
system and only one stream leaving. Thus, the energy balance simplifies to
1
2
2
1
2
2
v out
·
· Ĥ 1 v in 1 gh 2 m
·
05m
Ĥout 1
1 ghout 1 Q
in
in
out
in
2
2
(3.43)
· ) and leaving
Because this is a steady-state process, the mass flow rates entering (m
in
·
(mout) the process have to be identical; if they were not, the mass would be accumulating
· .
in the system. Going forward, we simply set both equal to m
The diagram implies that the boiler is oriented horizontally. As noted in Example 3-4,
if the entering and exiting streams are at the same height (hin 5 hout) and their mass flow
rates are identical, the potential energy terms cancel each other out. (See Example 3-6
for a system in which potential energy is significant.) The energy balance becomes
1
2
2
2
· Ĥ 1 v in 2Ĥ 2 v out 1 Q·
05m
in
out
2
2
(3.44)
Step 3 Obtain data
From the steam tables, Ĥ 5 640.1 kJ/kg for saturated liquid water at 5 bar and
Ĥ 5 3272.3 kJ/kg for steam at 5 bar and 4008C.
Step 4 Calculate unknown velocities
The velocities of the entering and exiting streams can also be found, because the sizes of
the pipes are known. First, the mass flow rate can be related to the volumetric flow rate
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 3 Material and Energy Balances
99
through the specific volume, which is also available in the steam tables:
1
·
· V̂ 5 10,000 kg
Vin 5 m
in
hr
2 10.001093 mkg 2 5 10.93 mhr
(3.45)
2 10.6173 mkg 2 5 6173 mhr
(3.46)
3
1
kg
·
Vout 5 m· V̂out 5 10,000
hr
3
3
3
The cross-sectional areas (A) of the pipes are
Ain 5 r 2 5 5 50.27 cm
18 cm
2 2
Aout 5 r 2 5 1 2 2 5 706.9 cm
2
30 cm 2
2
(3.47)
2
(3.48)
And the linear velocity (v) is equal to the volumetric flow rate divided by the crosssectional area:
1 hr
2 13600
s2
m
5
5 0.604
v 5
s
A
1m
s50.27 cm d 1
100 cm 2
(3.49)
16173 hr 2 13600 s2
m
v 5
5
5 24.26
s
A
1m
s706.9 cm d 1
2
100 cm
(3.50)
·
Vin
1
10.93
m3
hr
in
2
in
·
Vout
Typical vapor
velocities are much
larger than typical
liquid velocities.
2
m3
1 hr
out
2
out
2
Step 5 Solve energy balance
The energy balance expression can be rearranged as
3
1
v 2in v 2out
2
0 5 m· s Ĥin 2 Ĥout d 1
2
2
24 1 Q
·
(3.51)
Inserting known values gives
1
kg
0 5 10,000
hr
3
1
3
2 1
1 21
1N
kg ? m
s2
0 5 10, 000
kg
hr
2
kJ
kJ
2 3272.3
640.1
1
kg
kg
1J
N?m
4
1
1
0.604
2
2 1
m 2
s
2
24.26
2
m 2
s
2
2
Even though the
vapor velocity is
orders of magnitude
larger than the liquid
velocity, the exiting
stream kinetic energy
is only 0.3 kJ/kg
larger than that of the
incoming stream. By
contrast the change
in specific enthalpy is
over 2000 kJ/kg.
1 kJ
1Q
2 11000
J2
·
2 3122632.2 kgkJ 2 1 120.3 kgkJ 24 1 Q·
kJ
·
Q 5 2.632 3 10 7
hr
(3.52)
In Example 3-5, two distinct phenomena were identified and quantified:
1. The increase in enthalpy from the entering stream to the exiting stream.
2. The increase in kinetic energy from the entering stream to the exiting stream.
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100 Fundamentals of Chemical Engineering Thermodynamics
Physically, heat (Q) is added to the system, and this accounts for the higher energy of
the exiting stream. However, the kinetic energy term was insignificant compared to the
enthalpy term. This result is common in chemical process equipment. Going forward,
when we are examining standard chemical process equipment (e.g., heat exchangers,
pumps, turbines, compressors) we will assume kinetic energy of flowing streams is negligible unless we have a specific reason to think otherwise. Example 3-4 provides an example of a scenario in which kinetic energy cannot realistically be neglected: in a nozzle,
converting internal energy into kinetic energy is the entire purpose of the process.
The significance of potential energy is examined in the next example.
exaMPLe 3-6
geOtherMaLLy heated steaM
An underground reservoir 3 km below the surface contains geothermally heated steam
at T 5 2508C and P 5 5 bar. A vertical shaft allows this steam to flow upward, through
the shaft, and to the surface, where it enters a turbine. The effluent leaving the turbine is
saturated steam at P 5 1 bar (Figure 3-8).
WS 5 ?
mout 5 100 kg/min
Pout 5 1 bar, saturated steam
3 km
Q 5 25000 kJ/min
Tin 5 2508C
Pin 5 5 bar
Figure 3-8 Schematic of turbine/shaft described in Example 3-6.
The dashed line represents the boundary of the system as defined in
steps 2 and 3.
Determine the rate at which shaft work is produced by the turbine. Assume
The process operates continuously at steady state.
The flow rate of steam is 100 kg/min.
The turbine itself is adiabatic, but the steam loses 5000 kJ/min of heat to the surroundings as it travels upward through the shaft.
The steam velocity is negligible at the turbine exit and the shaft entrance.
SOLUTION:
Step 1 Define a system
In many problems, defining the system doesn’t seem like much of a decision, but in this
problem, there are several possible systems. At first glance, it seems natural to define the
turbine as the system, since the question asks how much work the turbine is producing.
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C H A P T E R 3 Material and Energy Balances
101
However, what happens when we do this? This is a steady-state system, so the energy
balance (Equation 3.34) immediately simplifies to
o5
j5J
05
1
·
Ĥj 1
m
j,in
j51
v 2j
2
26 o 5
1
26 1 W· 1 W· 1 Q· (3.53)
2
vk
·
m
Ĥk 1
1 ghk
k,out
2
k51
k5K
1 ghj
2
S
EC
·
·
The turbine is adiabatic (Q 5 0), and the system boundaries are not moving (WEC 5 0).
There is only one stream entering and one leaving, and at steady state, these must have
· 5m
· ). The energy balance further simplifies to
equal flow rates (m
in
out
1
2
2
1
2
2
·
· Ĥ 1 v out 1 gh
· Ĥ 1 v in 1 gh 2 m
1 WS
05m
in
out
in
out
2
2
(3.54)
The exiting velocity is explicitly stated in the problem statement as negligible, and based
on what we saw in Example 3-5, it would be logical to assume the same is true at the
turbine entrance. Figure 3-8 suggests the turbine is oriented horizontally, so we infer
the entering and exiting streams have equal potential energy (hin 5 hout). However, the
enthalpy of the stream entering the turbine (Ĥin) is unknown, so even if we make these
·
various simplifying assumptions, we cannot solve for the shaft work WS.
To determine Ĥin, we could do a second energy balance around the vertical shaft, knowing that the steam leaving the top of the shaft is the same as the steam entering the turbine.
However, this solution strategy is unnecessarily complicated. While no information is given
about the steam that enters the turbine, it is also the case that none is needed. In defining
·
a system, our purpose is to relate what we want to know (WS in the turbine) to what we do
know. The material entering the bottom of the shaft and the material leaving the turbine
are both fully described. Consequently, the most efficient solution strategy is to draw our
system boundary around both the shaft and the turbine, as shown in Figure 3-8. This way, we
have complete information about all of the material entering and leaving the system.
Step 2 Apply and simplify the energy balance
We use the time-dependent form of the energy balance because the given information
is in the form of rates. Since this is a steady-state process, the accumulation term is zero:
o5
j5J
05
1
·
Ĥj 1
m
j,in
j51
v 2j
2
26 o 5
2
1
26 1 W· 1 W· 1 Q· (3.55)
2
vk
·
m
Ĥk 1
1 ghk
k,out
2
k51
k5K
1 ghj
S
EC
There is only one stream entering and leaving, and both have negligible velocity and
· of 100 kg/min. Finally, there is no expansion/contraction work;
identical flow rates smd
the boundaries of the system are not moving. With these simplifications, the energy balance becomes
· sĤ 1 gh d 2 m· sĤ 1 gh d 1 W· 1 Q·
05m
(3.56)
in
in
out
S
out
Step 3 Solve the energy balance
·
·
The enthalpies of both streams are available in the steam tables. Q is given and WS is what
we are trying to determine. The only remaining unknowns are the values of h. We define
the surface of the Earth as h 5 0, so the steam exiting the turbine is at h 5 0 and the
steam entering the shaft is at h 5 23 km.
Inserting all known values produces
1
kg
0 5 100
min
2
3
19.8 ms2 s23000 md 1N ? m2 11000 J2
1J
kJ
2961.0
1
kg
2
kg ? m
s2
1
N
1 kJ
4
This is the energy
balance when the
turbine is defined as
the system.
PitFaLL
PreventiOn
Suppose we decided
to solve the problem
as described in this
paragraph, but the
kinetic energy of the
steam entering the
turbine in reality
wasn’t negligible.
Would our final
answer be wrong?
The velocity of the
steam at some places
inside the turbine is
undoubtedly quite
high, but that material
never crosses the
boundaries of the
system.
This is the energy
balance when the
system is defined as in
Figure 3-8.
Defining the reservoir
to be at h 5 0 and
the turbine to be at
h 5 13 km would
have been equally
logical and produced
the same answer.
(3.57)
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102 Fundamentals of Chemical Engineering Thermodynamics
PitFaLL
PreventiOn
Two common errors
in a calculation of
this kind would be: 1)
units; all terms must
be expressed in the
same units; in this case
kJ/min was used,
and 2) flipping the
·
sign for either W or
·
·
Q; in this case Q and
W are both negative
because the energy is
leaving the system.
exaMPLe 3-7
“Rigid” is a common
and useful key word.
You know that the
system is isochoric if
the boundaries of the
system cannot move.
Conceptually, you
could define “the
liquid” or “the vapor”
as the system, but if
you did that, you’d
have to consider it an
open system, because
material could
evaporate or condense
during this process.
1
2 100
1
0 5 100
kg
min
kg
min
2 32675.0 kg 1 04 1 W· 2 5000 min
kJ
kJ
S
232961.0 kg 1 1229.4 kg24 2 1100 min2 32675.0 kg 1 04 1 W· 2 5000 min
kJ
kg
kJ
kJ
kJ
S
220, 660
kJ
·
5 WS
min
This example is somewhat contrived in that it makes harvesting geothermal energy look
much simpler than it really is, but it illustrates a strategy for defining a system: choose a system that will allow you to use the known information and relate it to the information needed.
Example 3-6 illustrates a worthwhile point about potential energy. The process
had one material stream entering and one stream leaving, and the heights of these
two streams differed by 3 km. With Δ h 5 3 km, the potential energy term of the
energy balance accounted for ~30 kJ of energy per kilogram of steam. However, it is
not common for a chemical production process to involve heights of this magnitude.
If the difference in height between the entering and exiting streams had been 10 m (i.e.,
slightly more than the height of a typical three story house) instead of 3 km, the potential energy term would have been ~0.1 kJ/kg, which can typically be considered negligible compared to Ĥ terms that are on the order of hundreds or thousands of kJ/kg.
Consequently, going forward, we will treat the potential energy of entering and
exiting streams as negligible unless our system involves changes or differences in
height that are considerably larger than 10 m.
Examples 3-4 through 3-6 were steady-state, open-system processes. The next
example illustrates a closed system that is not at steady state.
cOOLing OF a t wO-phase Mixture
A rigid storage tank has a total volume of 5.00 m3 and is sealed throughout the process.
Initially, the storage tank contains 0.50 m3 of saturated liquid water at P 5 5 bar, while the
rest of the tank is filled with saturated steam at the same pressure. The tank gradually cools
until the liquid–vapor system is at P 5 4 bar (Figure 3-9). How much heat was lost to the
surroundings during this process, and what are the final volumes of liquid and vapor?
Initial Conditions
V1tot 5 5.00 m3
Final Conditions
V2tot 5 5.00 m3
Steam
Steam
P1 5 5 bar
V 1v 5 4.5 m3
P2 5 4 bar
V 2v 5 ?
V1L 5.5 m3 P1 5 5 bar
V2L 5 ? m3 P2 5 4 bar
Figure 3‑9 Schematic of initial and final conditions of tank described in
Example 3-7.
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C H A P T E R 3 Material and Energy Balances
103
SOLUTION:
Step 1 Define a system
Define the contents of the tank (both liquid and vapor) as the system. The tank is sealed,
so it is a closed system. While the total mass of the system must remain constant, the
masses of liquid and vapor phases can change.
Step 2 Apply and simplify the energy balance
We use the time-independent energy balance since no information regarding duration of
the process is known. This is not a steady-state process; we know the pressure is changing
and must expect that other properties (like U) will also be changing. There are, however,
several immediate simplifications to the energy balance:
There is no mass entering or leaving the system (it is a closed system).
Potential and kinetic energy portions of the “accumulation” term can be ignored; the
tank is not moving.
The tank is rigid, so there can be no work of expansion or contraction.
There is no mention of a mixer or any other mechanism for shaft work in the tank.
Thus, the energy balance simplifies to
D sMÛd 5 Q
(3.58)
U2 2 U1 5 Q
(3.59)
Or, equivalently:
U 5 MÛ by the
definition of specific
internal energy
Heat is the only form in which energy is added to or removed from the system. We expect Q
to be negative, since it is stated that the tank “cools”; this implies energy is leaving the system.
Step 3 Quantify initial liquid and vapor masses
Step 2 shows we cannot find Q without determining the initial (U1) and final (U2) values
of internal energy, which requires determining how much liquid and vapor are present at
the beginning and end of the process.
According to the steam tables, the specific volumes of saturated liquid and vapor
at P 5 5 bar are V̂L 5 0.001093 m3/kg and V̂V 5 0.3748 m3/kg. Consequently, the initial
masses of liquid and vapor can be found as
M L1 5
V L1
5
Vˆ L
1
M 1V 5
0.50 m3
5 457.5 kg
m3
0.001093
kg
V 1V
5
Vˆ V
1
4.5 m3
5 12.0 kg
m3
0.3748
kg
(3.60)
(3.61)
Recall the Gibbs
phase rule:
F 5 C 2 π 1 2. With
one component and
two phases, there
is only one degree
of freedom. When
pressure is set (P 5
5 atm), there are no
degrees of freedom
remaining; any other
intensive property
of the system (e.g.,
specific volume) must
have a unique value.
Thus, the total mass is 457.5 1 12.0 5 469.5 kg.
Step 4 Quantify final liquid and vapor masses
The amounts of liquid and vapor at the end of the process (M L2 and M V2 ) are unknown, so
we need two equations to determine them. One equation comes from the fact that this is
a closed system; total mass is constant:
M L2 1 M V2 5 469.5 kg
(3.62)
The other equation comes from the fact that the container is rigid, so the total volume
must remain 5.0 m3.
M L2 V̂ L2 1 M V2 V̂ V2 5 5.0 m3
(3.63)
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104 Fundamentals of Chemical Engineering Thermodynamics
Saturated liquid and vapor volumes at 4 bar are again available in the steam tables. Thus,
1
M L2 0.001084
2
1
2
m3
m3
1 M V2 0.4624
5 5.0 m3
kg
kg
(3.64)
Solving these two equations simultaneously gives M2L 5 459.8 kg and M2V 5 9.7 kg. The
final volume of liquid is 0.498 m3. Recall that the original volume was given with two
significant figures, 0.50 m3, so to this level of accuracy, the liquid and vapor volumes are
unchanged. This result is plausible, because approximately 20% of the original vapor has
condensed, but the remaining vapor is significantly less dense than the original vapor due
to the decrease in pressure.
Step 5 Solve energy balance for Q
Returning to the energy balance, we know ΔU 5 Q and that the total internal energy
at either the beginning or end of the process is the sum of the liquid and vapor internal
energy. Thus,
Q 5 U2 2 U1 5 sM L2 Uˆ L2 1 M 2V Uˆ 2V d 2 sM L1 Uˆ 1L 1 M V1 Uˆ 1Vd
(3.65)
The masses have all been determined and the internal energy of saturated liquid and
vapor at P 5 5 bar and P 5 4 bar can be obtained from the steam tables. Thus,
3
1
Q 5 s459.8 kgd 604.2
3
1
2
1
kJ
kJ
1 s9.7 kgd 2553.1
kg
kg
2 s457.5 kgd 639.5
2
24
1
kJ
kJ
1 s12.0 kgd 2560.7
kg
kg
24
(3.66)
Q 5 220, 723 kJ
We have now examined examples of open and closed systems, as well as examples of
steady-state and unsteady-state processes. This section closes with an example where
the shaft work required in a pump is determined. The following example combines
the results of an open system, steady-state energy balance with the results of a closed
system, unsteady-state energy balance.
exaMPLe 3-8
shaFt wOrk in a water puMp
An adiabatic pump operates at steady state. Water enters as saturated liquid at P 5 0.2 bar
and is compressed to P 5 10 bar (Figure 3-10). What is the rate at which shaft work is
added in the pump, per kilogram of entering water?
Saturated
liquid water
Pin 5 0.2 bar
Pout 5 10 bar
WS/m 5 ?
Figure 3‑10 An adiabatic pump operating at steady
state, described in Example 3-8.
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C H A P T E R 3 Material and Energy Balances
SOLUTION:
Step 1 Define a system
The system is defined as a pump with the system boundaries shown in Figure 3-10.
105
In steps 1 and 2, the
system is the pump.
Step 2 Apply and simplify the energy balance
Because the pump operates at a steady state, the left-hand side of the energy balance
is 0. Thus,
v 2j
o 5m· 1Ĥ 1 2 1 gh 26 2 o 5m·
j5J
05
j,in
j
k5K
j
j51
k51
1
Ĥk 1
k,out
v 2k
2
26 1 W· 1 W· 1 Q·
1 ghk
S
EC
There is only one stream entering and one stream leaving, the boundaries of the system
·
·
are not moving (WEC 5 0), and the pump is described as adiabatic (Q 5 0). Neglecting
potential and kinetic energy leaves the energy balance:
· Ĥ 2 m
· Ĥ 1 W·
(3.67)
05m
in
in
out
out
S
The mass flow rates entering and leaving are equal, so the “in” and “out” subscripts can
be eliminated from the mass flow rates. It is convenient to rearrange the equation so that
it is explicit in “work produced per kilogram of water,” since that it what we are trying
to determine. Thus,
·
WS
(3.68)
· 5 Ĥout 2 Ĥin
m
PitFaLL
PreventiOn
The given information
says the liquid
entering the pump is
saturated, but there is
no reason to expect
that the liquid exiting
the pump is also
saturated.
At this point, however, there is no clear way to progress with Equation 3.68. We know
exactly what is entering the pump—saturated liquid at P 5 0.2 bar—and can look up Ĥin
in the steam tables. We know the exiting fluid is at P 5 10 bar, but as the Gibbs phase
rule states, it takes two intensive properties to fully specify the state of a pure fluid, and
we have no second piece of information. We can gain some deeper insight into what’s
happening in this process through a second energy balance.
Step 3 Define a second system
The only plausible systems we could define are the pump, examined in step 2, and the
water itself. We can define a small “slug” of liquid as the system, with the process beginning right before the slug enters the pump and ending when the liquid leaves the pump.
We will model this “slug” of liquid as a closed system; thus we assume it does not mix
with the water surrounding it. This system is illustrated in Figure 3-11.
Saturated
liquid water
Pin 5 0.2 bar
Initial state
of system
In steps 3 through 6,
the system is a small
slug of liquid water,
and the process is its
passage through the
pump.
Pout 5 10 bar
WS/m 5 ?
Final state
of system
Figure 3‑11 Alternative approach to modeling the pump, where the system
is a slug of fluid.
Step 4 Apply and simplify the energy balance
Since we don’t know how long it takes a slug of liquid to pass through the pump, we apply
the time-independent energy balance, which in its most general form is
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106 Fundamentals of Chemical Engineering Thermodynamics
5 1
D M Û 1
v2
1 gh
2
v 2j
26 o 5 1
j5J
mj, in Ĥj 1
5
2
j51
o5
k5K
2
k51
26
1 ghj
1
mk,out Ĥk 1
v 2k
2
26 1 W 1 W 1 Q
1 ghk
EC
S
Since we know that at least one property of the system (pressure) is changing, this is not
a steady-state process; ΔU on the left-hand side cannot be neglected. But we will neglect
potential and kinetic energy for reasons that were explored in Examples 3-5 and 3-6.
On the right-hand side, the terms associated with material flow can be eliminated,
since the slug is a closed system. The pump as a whole is adiabatic, so we expect there is
also no heat transfer into this small slug of liquid, thus, Q 5 0. In addition, by this definition of the system, shaft work is zero. This is because:
When the pump, and everything inside the pump, is the system, the machinery is
working inside the boundaries of the system and transfers energy in the form of
shaft work.
When a small slug of liquid is the system, there are no moving parts inside the system. The shaft is outside the system, but it acts on the liquid, compressing it. Thus, by
this definition of the system, the work done is work of expansion/contraction.
The energy balance thus simplifies to
DsMÛd 5 WEC
(3.69)
The mass of the slug of liquid is constant, because this is a closed system. The specific internal energy is not constant. While we don’t know exactly how the pump works, we know
the initial and final values of Û are those of the liquid entering and leaving the pump: the
process begins when the fluid enters the pump and ends when the liquid leaves the pump.
M sÛout 2 Ûin d 5 WEC
(3.70)
Step 5 Relate WEC to intensive properties of liquid
We were unable to solve the problem in step 2 because we knew only one intensive property (P) of the liquid exiting the pump. To be useful, Equation 3.70 must also be related
to intensive properties of the liquid. We learned in Section 1.4.3 that expansion work is
equal to –P dV. Thus,
By definition, V̂ 5
V
M
M sÛout 2 Ûin d 5
#
Vout
2 P dV
(3.71)
2 P dV̂
(3.72)
Vin
Dividing both sides by the system mass M gives
sÛout 2 Ûin d 5
#
Vˆout
Vˆin
At this point, we appear to have a similar barrier to further progress as we had in step 2.
The initial state of the water is fully specified and we can look up V̂in in the steam tables,
but we don’t have any information about the liquid leaving the pump that would help us
resolve the integral. However, the connection between Equations 3.72 and 3.68 becomes
more apparent when we use integration by parts.
Step 6 Integration by parts
Integration by parts yields
sÛout 2 Ûin d 5 2PV̂ u
out
in
1
# V̂ dP
Pout
(3.73)
Pin
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C H A P T E R 3 Material and Energy Balances
sÛout 2 Ûin d 5 PinV̂in 2 PoutV̂out 1
# V̂ dP
Pout
(3.74)
Pin
107
Get into the habit of
recognizing U 1 PV
as H.
But we can combine the U and PV terms, applying the definition of enthalpy for
sÛout 1 PoutV̂out d 2 sÛin 1 PinV̂in d 5
Ĥout 2 Ĥin 5
#
Pout
V̂dP
Pin
# V̂dP
Pout
(3.75)
Pin
Step 7 Combine energy balance equations
Equations 3.68 and 3.75 were derived through two different energy balances on two
different systems, but both relate the change in enthalpy experienced by the liquid as it
travels through the pump: Ĥout 2 Ĥin. Combining these expressions gives
·
WS
· 5
m
# V̂dP
Pout
(3.76)
Pin
Step 8 Solve for pump work
There are two key points about Equation 3.76 that make it solvable. First, the integral is
with respect to dP rather than dV̂, and P is the one property that is known for both the
entering and exiting water. Second, the specific volume of a liquid (under most conditions) is reasonably approximated as constant. Applying this gives
·
WS
(3.77)
· < V̂ sPout 2 Pin d
m
·
N
WS
m3
J
10 5 Pa 1
1J
< 0.001017
5 996.7
s10 2 0.2 bard
m2
·
m
kg
bar
N?m
kg
Pa
1
2
21
1
21
The specific volume
used here is that of
saturated liquid at
P 5 0.2 bar, because
that is the only state of
the water that is fully
specified.
2
In this example, it only takes about 1 kJ of work to increase the pressure of 1 kg of
water by a factor of 50.
A key result from Example 3-8 is Equation 3.76, which relates the shaft work in a
pump to the volume and pressure of the fluid flowing through the pump. Multiplying
both sides of Equation 3.76 by mass flow rate gives
·
WS 5
#
P5Pout ·
VdP
(3.78)
You can’t integrate
Equation 3.78 unless
·
you can express V as
a mathematical
function of P.
P5Pin
The derivation of this equation is not specific to a pump; in principle it could be
applied to a compressor, a turbine, or any other process involving shaft work.
In practice, it is very difficult to apply Equation 3.78 to a process in which the
fluid is a vapor (or liquid–vapor system.) A vapor in a turbine or compressor
undergoes large changes in both pressure and temperature. One would need a
·
very detailed model of the process in order to relate the volumetric flow rate V
to pressure P, and without this relationship one cannot integrate Equation 3.78.
Turbines and compressors are commonly modelled with strategies introduced in
Chapter 4.
Equation 3.78 is quite useful for pumps, however, because in many applications,
liquid volumes are reasonably modeled as constant with respect to T and P, as was
done in Equation 3.77 in Example 3-8.
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108 Fundamentals of Chemical Engineering Thermodynamics
3.5 combining the energy balance with simple thermodynamic Models
While the physical systems in the examples found in Section 3.4 were varied, the only
chemical compound present in any of them was H2O. Application of the energy balance
was therefore simple in one respect: Û, Ĥ, and V̂ were known at all of the conditions
of interest. In this section, we will examine the application of the energy balance in
conjunction with the simple thermodynamic models developed in Section 2-3, which is
crucial in situations where physical property data is less extensive or not available at all.
exaMPLe 3-9
heLiuM-FiLLed baLLOOn
The balloon portion of a dirigible contains 5000 moles of helium. Initially, the helium is
at T 5 158C and P 5 0.95 atm, and is 500 m off the ground (where the atmospheric pressure is also 0.95 atm). The dirigible flies south at a constant height and a slow velocity, and
gradually warms to T 5 258C (though the pressure remains constant) (Figure 3-12). How
much heat was added to the balloon during the process?
Initial Dirigible
Final Dirigible
5000 moles He; P 5 0.95atm; T1 5 158C
5000 moles He; P 5 0.95atm; T2 5 258C
500 m
FOOd FOr
thOught
3-15
Try simplifying the
energy balance
equation yourself
before reading step 2.
pitFaLL
preventiOn
The fact that pressure
is constant does not
indicate the process
is at steady state.
Steady state means
ALL properties
of the system are
constant with respect
to time, and in this
case, the temperature
of the system is
changing.
Figure 3‑12 Schematic of dirigible flight described in Example 3-9.
SOLUTION:
Step 1 Define the system
The given information is all in the form of properties of the helium so define the helium
as the system.
Step 2 Apply and simplify the energy balance
We use the time-independent energy balance because there is no information on how
long the flight took.
This is a closed system; no mass enters or leaves. Thus, min 5 mout 5 0, and the energy
balance simplifies to
5 1
D M Û 1
v2
1 gh
2
26 5 W 1 W 1 Q
EC
S
(3.79)
We know Q is significant because it is what we are trying to find. One likely error, however,
would be crossing out both of the work terms. Because producing work is not the apparent
purpose of the process, it would be easy to fall into the trap of assuming that no work occurs. There are no moving parts inside of a balloon, so no shaft work is produced. However,
the gas is being heated at constant pressure, so it must expand, and anytime the system is
expanding, work is being done on the surroundings (as explained in Section 1.4.3).
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C H A P T E R 3 Material and Energy Balances
On the left-hand side, kinetic energy can be neglected, since velocity (v) is described
as “slow.” The height h is constant at 500 m, so the potential energy relative to the ground
is not 0, but it is constant. Thus, the energy balance simplifies to
M2Û2 2 M1Û1 5 WEC 1 Q
(3.80)
Because the number of moles of helium is given, it’s more convenient to express this on
a molar basis:
N2 U 2 2 N1 U 1 5 WEC 1 Q
(3.81)
Thus, if we can evaluate any two of heat, work and molar internal energy, we can compute
the third with the energy balance. Note the amount of helium doesn’t change, so N2 5 N1 5 N.
Step 3 Find expansion work
As derived in Section 1.4.3, work of expansion/contraction is given by
dWEC 5 2P dV
(3.82)
where P is the pressure opposing the motion. In this case, the external pressure is from
the atmosphere. However, because of the elevation, the “external pressure” is 0.95 atm,
not 1 atm. As the external pressure is constant, Equation 3.82 integrates to
WEC 5 2s0.95 atmd sV2 2 V1d
(3.83)
Since the gas pressure is below atmospheric, we will assume ideal gas behavior. V2 and V1
can be found with the ideal gas law as
WEC 5 2s0.95 atmd
1P 2 P 2
NRT2
NRT1
2
1
(3.84)
In this case P1 5 P2 5 0.95 atm, so the equation simplifies. (The fact that the gas pressure
and external pressure are equal is discussed further at the end of the example.)
WEC 5 2NR sT2 2 T1d
1
WEC 5 2s5000 molesd 8.314
2
(3.85)
1
2
J
1 kJ
s298.15 2 288.15 Kd
5 2415.7 kJ
mol ? K
1000 J
The “accumulation”
term quantifies
changes in the energy
of the system. If height
is constant (h2 5 h1)
then there is no change
in potential energy.
Equation 3.80
represents a common
special case: a simple,
closed system with no
moving parts and no
significant changes in
kinetic or potential
energy.
U 5 MÛ 5 NU by the
definitions of specific
and molar internal
energy
Example 2-3
showed that water
vapor is reasonably
approximated as
an ideal gas at
pressures up to
P 5 1 atm. The ideal
gas approximation is
even more realistic for
helium than for H2O
at these low pressures,
because the noble
gases are non-polar.
Step 4 Calculate change in molar internal energy
In Section 2.3.3, we showed that for an ideal gas
dU 5 CV* dT
(3.86)
Furthermore, helium is a monatomic gas. As discussed in Example 2-4, we can assume
CV* 5 (1.5)R for any monatomic ideal gas. Since CV* is constant, Equation 3.86 integrates to
U 2 2 U 1 5 CV* sT2 2 T1d 5
1
2
3
J
J
8.314
s298.15 2 288.15 Kd 5 124.7
(3.87)
2
mol ? K
mol
Step 5 Solve energy balance for Q
Inserting the results of steps 3 and 4 into the energy balance gives
1
s5000 molesd 124.7
J
mol
1 kJ
5 2415.7 kJ 1 Q
2 11000
J2
109
(3.88)
Q 5 1039.2 kJ
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110 Fundamentals of Chemical Engineering Thermodynamics
A possible source of confusion
According to the given information, the atmosphere and the helium have constant
P 5 0.95 atm. You may ask, if the inside and outside pressures are identical, how can the
expansion occur in the first place?
The key word in the problem statement is “gradually.” The heat is added very slowly,
such that the pressure inside the balloon never climbs significantly higher than the
external pressure. Since the pressure difference driving the expansion is very small, the
expansion is very slow. Consequently, even though we know the helium pressure must
be fractionally higher than the external pressure (at least part of the time) in order for
the expansion to occur, in building a mathematical model of the process, we can treat
these pressures as identical. This is actually an example of a reversible process, a phenomenon discussed in detail in Chapter 4.
The next example again examines a gas confined in a piston-cylinder device. It is
instructive to note that, while the physical system looks very different from the dirigible in Example 3-9, when we go through the exercise of identifying relevant forms
of energy entering, leaving, or accumulating in the system, the energy balance equation ends up looking substantially the same.
exaMPLe 3-10
ideaL gas in a pistOn-cyLinder device
An ideal gas has CP* 5(7/2)R. One mole of this gas is confined in a piston-cylinder
device. Initially, the gas is at T 5 300 K and P 5 1 bar (Figure 3-13). If the gas is compressed isothermally to P 5 5 bar, find the amounts of work and heat associated with
the process.
Initial State
n1 5 1 mol
T1 5 300 K
P1 5 1 bar
Final State
n2 5 1 mol
T2 5 300 K
P2 5 5 bar
Figure 3‑13 Piston-cylinder device described in Example 3-10.
SOLUTION:
Step 1 Define the system
Define the system as the gas inside the cylinder.
Step 2 Apply and simplify the energy balance
There is no information given or needed on how long the process takes, so we use the
time-independent form of the energy balance. Note the following simplifications.
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C H A P T E R 3 Material and Energy Balances
This is not a steady-state process; the accumulation term as a whole can’t be
neglected. However, the cylinder, as a unit, is not moving through space, and
changes in the kinetic or potential energy of the system can be considered
negligible.
There is no material entering or leaving the system, min 5 mout 5 0.
While a piston may have moving/rotating parts, these are outside the system. There
are no moving parts inside the cylinder so WS 5 0.
The system gets smaller so WEC is not negligible.
There is no reason to assume Q is negligible.
Consequently, the energy balance simplifies, as it did in Example 3-9, to
N2 U 2 2 N1 U 1 5 WEC 1 Q
(3.89)
The internal energy here is expressed on a molar (rather than a mass) basis.
Step 3 Determine work of compression
The work can be determined using Equation 1.22:
111
A piston-cylinder
device is a very
different looking piece
of equipment than
a balloon, but they
have the same energy
balance equation; the
forms in which energy
enters, leaves, and
accumulates are the
same in both systems.
Whenever a system
is expanding or
contracting, work
occurs, and can be
quantified using
Equation 1.22.
dWEC 5 2P dV
where P is the pressure opposing the motion—in this case, the gas pressure. Solving the
ideal gas law for P gives
P5
NRT
V
(3.90)
Introducing the ideal gas law into Equation 1.23 gives
dWEC 5 2
NRT
dV
V
(3.91)
A crucial difference
between this example
and Example 3-9 is
that P is not constant.
In order to integrate
with respect to V, we
must therefore express
P as a mathematical
function of V.
This is stated as an isothermal compression, and the system is closed. Therefore, T and N
are both constant in this process, and R is always a constant. Consequently, Equation 3.91
can be integrated as
WEC 5 2NRT
#
V5V2 1
V5V1 V
dV 5 2NRT s ln V2 2 ln V1d 5 2NRT ln
1V 2
V2
(3.92)
1
N, R, and T are all known. Because the initial and final pressures and temperatures are
given, we could certainly determine V1 and V2, but it is not actually necessary to do so. We
can instead use the ideal gas law to express V1 and V2 in terms of the known temperatures
and pressures:
1 2
NRT2
WEC 5 2NRT ln
P2
NRT1
(3.93)
P1
Because N, R, and T are all constant (T1 5 T2 and N1 5 N2), Equation 3.93 simplifies to:
WEC 5 2NRT ln
1P 2 5 2s1 mold 18.314 molJ? K2 s300 Kd ln 115 bar
bar 2
P1
It makes sense that
W is positive in this
case. Work is being
done on the system
by the surroundings,
which means energy is
added to the system.
(3.94)
2
5 4014 J
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112 Fundamentals of Chemical Engineering Thermodynamics
Step 4 Determine change in molar internal energy
There is no direct way to compute Q, but now that we know WEC, if we can quantify the
change in internal energy, we can compute Q from the energy balance. Section 2.3.3 indicated that for an ideal gas:
dU 5 CV* dT
This is an isothermal process (dT 5 0). Thus, even though this is not a steady-state process, the change in internal energy is 0. Consequently the energy balance simplifies to
CV dT 5 0 5 WEC 1 Q 5 4014 J 1 Q
(3.95)
24014 J 5 Q
Note several points illustrated by the previous example:
The expression
CP* 5 CV* 1 R is
specifically valid for
ideal gases and is not
valid in general.
A process is at steady state if ALL state variables remain constant with time. In
the example, the temperature and molar internal energy of the system were both
constant, but the pressure was changing, so the system was not at steady state.
For an ideal gas, internal energy is only a function of temperature. It would have
been reasonable, at the start of the problem, to recognize that internal energy
does not change because temperature does not change, and cross out the U term
in the generalized energy balance for that reason. However, the solution demonstrated that even if one had missed this physical insight at the start of the
problem, Equation 3.95 would still lead to the correct answer.
CV* appears in Equation 3.95, but its value doesn’t matter because dT 5 0. Note,
however, that CP* was given, and it was shown in Section 2.3.3 that for an ideal
gas, CV* 1 R 5 CP*. Therefore, in this case, had CV* been needed, one could have
computed it:
7
5
C V* 5 C P* 2 R 5 R 2 R 5 R
2
2
The previous two examples concerned ideal gases. The next problem involves a
liquid, which will be modeled using the assumptions discussed in Section 2.3.5.
Example 3-11 also requires use of the time-dependent energy balance.
exaMPLe 3-11
A system can be
adiabatic either
because (1) it is so
well insulated that
there is essentially
no heat transfer,
as in this problem
or (2) because
the system and
surroundings are at
the same temperature
throughout the
process; there is no
driving force for heat
transfer.
a stOrage tank
A tank has a volume of 500 liters and is initially full of a liquid that is at 300 K, has a density of 0.8 kg/L, and a constant heat capacity of ĈV 5 3 J/g · K. The tank can be modeled as
perfectly insulated, and its contents can be assumed to be well mixed, although the shaft
work added by the mixer can be considered negligible.
The inlet and outlet valves are both opened at the same time. The flow rates of the
entering and leaving liquids are both 25 L/min. The entering liquid is at T 5 400 K, and
the temperature of the exiting liquid at any time is identical to the temperature of the
liquid in the tank at that particular time (Figure 3-14). After five minutes, what is the
temperature of the liquid in the tank?
SOLUTION:
Step 1 Define a system
Define the contents of the tank as the system.
Step 2 Apply the material balance
There is no information in the problem statement that would let us quantify how much
the liquid expands with increasing temperature—we don’t even know what the liquid is,
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C H A P T E R 3 Material and Energy Balances
V 5 25 L/min
Tin 5 400 K
FOOd FOr
thOught
3-16
What would happen
to a real tank filled
with liquid if it
was heated and
there was no space
above the liquid to
accommodate the
thermal expansion?
V 5 500 L
T0 5 300 K
r 5 0.8 kg/L
^
Cv 5 3 J/g ∙ K
T5?
V 5 25 L/min
Tout 5 T 5 ?
Figure 3‑14 Tank described in Example 3-11.
specifically. We do know the density of liquids (and solids) is frequently, and to a reasonable approximation, constant with respect to temperature, so we will assume the density
0.8 kg/L (given at T 5 300 K) is valid at all temperatures.
Consequently, the mass of liquid entering and leaving the tank is
1
· 5m
· 5 25 L
m
out
in
min
2 10.8 L 2 5 20 min
kg
kg
(3.96)
Because the entering and exiting mass flow rates are identical, the mass balance
simplifies to
dM
· 50
· 2m
5m
out
in
dt
(3.97)
Thus, the mass of the system (M) is constant with respect to time and can be computed as
1
M 5 s500 Ld 0.8
2
kg
5 400 kg
L
(3.98)
Step 3 Apply and simplify the energy balance
5 1
113
v2
d
M Û 1
1 gh
dt
2
v 2j
26 o 5 1 2 1 gh 26
v
·
2 o 5m
1Ĥ 1 2 1 gh 26 1 W· 1 W· 1 Q·
j5J
5
·
m
Ĥj 1
j, in
j
j51
2
k
k5K
k, out
k
k
S
EC
k51
Examining the terms from left to right:
Û is not constant, since we expect the tank temperature to increase. However, the
system is a stationary tank, so there is no change in kinetic or potential energy.
There is one stream entering and one leaving the process. The kinetic and potential
energies of these streams can be neglected (see Examples 3-5 and 3-6).
Shaft work is explicitly stated as negligible.
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114 Fundamentals of Chemical Engineering Thermodynamics
FOOd FOr
thOught
3-17
Section 3.4 showed
that 10 m of height is
equivalent to ~0.1 kJ
of potential energy
per kilogram of mass.
What are realistic
dimensions for a 500 L
tank? Is the height
likely to be more or
less than 10 m?
The steam tables show
that for saturated
liquid water at 25°C,
kJ
Û 5 104.8 , and
kg
kJ
Ĥ 5 104.8 ; these
kg
are identical to four
significant figures.
The saturated liquid
values start to diverge
as temperature and
pressure increase, so
at T 5 175°C and
P 5 8.926 bar, Û and Ĥ
differ by 1.0 kJ/kg.
There is no work of expansion or contraction since the system is a constant-volume
vessel.
Heat can be considered negligible since the vessel is described as perfectly insulated.
Applying these simplifications gives the energy balance:
d
· Ĥ
· Ĥ 2 m
sMÛ d 5 m
out
in
in
out
dt
Since M is constant it can be pulled out of the differential. Also, m· in 5 m· out 5 m· , so
M
dÛ
5 m· sĤin 2 Ĥout d
dt
Heat capacity can be
measured on either a
molar or mass basis.
Here ĈV 5 3 J/g · K, so
we write dÛ 5 ĈV dT.
In Example 3-10, CV
was given on a molar
basis, so we wrote
dU 5 CV dT.
(3.100)
Step 4 Simplifying assumption regarding enthalpy
It was observed in Section 2.3.5 that for most liquids and solids, PV << U, so U < H and
CV < CP. Since we have no better way to progress using the given information, we assume
U 5 H (and Û 5 Ĥ) here.
M
dÛ
5 m· sÛin 2 Ûoutd
dt
(3.101)
Step 5 Relate specific internal energy to known heat capacity
This is a constant-volume process (see Section 2.3.2), and for a constant-volume process:
dÛ 5 ĈV dT
(3.102)
We can relate the term Ûin 2 Ûout to temperature by integrating Equation 3.102. Since
ĈV is constant,
#
ˆ
ˆ
U5U
out
ˆ
ˆ
U5U
“The given
information” is an
artificial constraint in
textbook problems.
In engineering
practice, you need to
consider whether the
uncertainty introduced
by your assumptions
is acceptable, and you
always have the option
of seeking additional
information.
(3.99)
dÛ 5
in
#
T5Tout
T5Tin
ĈV dT
(3.103)
Ûout 2 Ûin 5 ĈV sTout 2 Tin d
Substituting this and Equation 3.102 into Equation 3.101 gives
MĈV
dT
· Ĉ sT 2 T d
5 2m
V
out
in
dt
(3.104)
Step 6 Solve differential equation
Because the tank is well mixed, the exiting liquid temperature (Tout) is the same as the
temperature of the system (T). Making this substitution, and also cancelling ĈV, gives an
equation we can integrate:
M
dT
· sT 2 T d
5m
in
dt
·
dT
m
5 dt
Tin 2 T M
(3.105)
(3.106)
·
t5t m
dT
5
dt
T5300 K Tin 2 T
t50 M
#
#
T5T
2 ln
1T 2 300 K2
Tin 2 T
in
5
·
m
t
M
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 3 Material and Energy Balances
Substituting in known values:
PitFaLL
PreventiOn
kg
20
400 K 2 T
min
5
t
2ln
400 K 2 300 K
400 kg
1
2
T 5 400 K 2 s100 Kde[2s0.05 min dt]
21
(3.107)
T 5 322.1 K at a time of 5 minutes, and a full graph of T versus t is shown in Figure 3-15.
500
Temperature (K)
115
Mixing up the two
temperatures is
a common error
when relating U to
T through the heat
capacity. Writing out
“Tfinal 2 Tinitial” or
“Tout 2 Tin” instead of
simply “ΔT ” can help
you avoid such errors.
400
300
200
100
0
0
20
10
30
Time (min)
40
50
60
The temperature
in the tank
asymptotically
approaches the
temperature of the
entering liquid, as
shown in Figure 3-15.
Figure 3‑15 Temperature versus time for the water in the tank in
Example 3-11.
Up to this point, the examples in this section all involved a single phase (gas in
Examples 3-9 and 3-10, liquid in Example 3-11). The next example illustrates a
steady-state process that involves both a phase change and a temperature change.
steady-state bOiLer
exaMPLe 3-12
1000 mol/min of pure ethanol enters a steady-state boiler as liquid at P 5 1 atm and
T 5 258C, and leaves the boiler as vapor at P 5 1 atm and T 5 1008C as shown in Figure 3-16.
The normal boiling point of ethanol is 78.48C. At what rate is heat is added to the boiler?
Q5?
Pin 5 1 atm
Tin 5 258C
nin 5 1000 mol/min
liquid
Pout 5 1 atm
Tout 5 1008C
vapor
Figure 3‑16 Schematic of ethanol boiler modeled in Example 3-12.
SOLUTION:
Step 1 Apply and simplify the energy balance
The system is the boiler and its contents. Since this is a steady-state process, the accumulation term of the energy balance is zero:
1
2
2
1
2
2
v out
·
·
·
· Ĥ 1 v in 1 gh 2 m
·
05m
Ĥout 1
1 ghout 1 Q 1 WS 1 WEC
in
in
out
in
2
2
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116 Fundamentals of Chemical Engineering Thermodynamics
There is no mechanism for work in a typical heat exchanger, and neglecting potential and
kinetic energy changes produces:
· sĤ d 2 m
· sĤ d 1 Q·
05m
in
in
out
out
the entering and exiting mass flow rates are equal so:
·
0 5 m· sĤ 2 Ĥ d 1 Q
in
out
The given information is on a molar basis so we switch to this (mĤ 5 nH ) and solve for
rate of heat addition:
·
(3.108)
Q 5 n· sH 2 H d
out
in
Step 2 Formulate strategy for computing H out 2 H in
A common error would be to recognize that this is a constant-pressure process and simply apply CP :
H out 2 H in 5
#
Tout 5 1008C
Tin 5 258C
CP dT
and look up the heat capacity of ethanol in Appendix D-1 or D-2. But a closer look at Appendices D-1 and D-2 reveals that ethanol liquid and ethanol vapor have different heat
capacities. More fundamentally, dH 5 CP dT accounts only for temperature changes and
does not account for the change in molar enthalpy that accompanies the phase change.
Instead, we find H out 2 H in by applying the fact that molar enthalpy is a state property:
The change in molar enthalpy is independent of path. We envision a three-step path, illustrated in Figure 3-17; step 1 is the heating of the liquid to its normal boiling point, step 2 is
the phase change, and step 3 is the heating of saturated vapor to 1008C. The molar enthalpy
changes for these individual steps sum to the molar enthalpy change for the whole path:
H out 2 H in 5 D H 1 1 D H 2 1 D H 3
State of entering
ethanol
Liquid
Pin 5 1 atm
Tin 5 258C
State of exiting
ethanol
Vapor
Pout 5 1 atm
Tout 5 1008C
Actual Process
DH 5 H out 2 H in
DH 1
Liquid
P 5 1 atm
T 5 78.48C
(3.109)
DH 3
Vapor
P 5 1 atm
T 5 78.48C
DH 2
Figure 3‑17 Calculation of change in molar enthalpy for ethanol in Example 3-12.
Step 3 Calculate Δ H for individual steps
Step 2 is boiling at atmospheric pressure, so Δ H 2 is simply Δ H vap for ethanol at atmospheric pressure, which is available in Appendix C.
D H 2 5 36, 560
kJ
mol
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C H A P T E R 3 Material and Energy Balances
Steps 1 and 3 are both temperature changes at constant pressure, so dH5CP dT applies
to each, as discussed in Section 2.3.2.
DH 1 5
#
T 5 78.4 8C
DH 3 5
#
T 5 100 8C
C P , liq dT
T 5 25 8C
T 5 78.4 8C
117
The integration
of a temperaturedependent expression
for CP was illustrated
in Example 2-5.
C *P dT
Note that CP* is the ideal gas heat capacity of ethanol, which is available in Appendix C-1.
Because this process is carried out at a low pressure (1 atm), it is reasonable to model
ethanol vapor as an ideal gas, as discussed in Section 2.3.3. The value of CP for liquid
ethanol is available in Appendix C-2. Both heat capacities are functions of T. The full
integration of CP dT is shown in the more detailed solution provided in the electronic
appendices.
DH 1 5 6253 Jymol
DH 3 5 1621 Jymol
Step 4 Calculate Q
Returning to Equation 3.108 to solve for the heat addition:
·
Q 5 n· sH 2 H d
out
1
mol
·
Q 5 1000
min
in
1 kJ
kJ
5 44,400
2 16253 1 36, 560 1 1621 molJ 2 11000
J2
min
Notice the molar
enthalpy of
vaporization is much
larger in magnitude
than the changes
in molar enthalpy
resulting from the
temperature changes.
The previous examples illustrate the process of applying and simplifying the generalized energy balance equation to a variety of physical systems. We close this section
by considering a rocket launching, which revisits the motivational example in a more
quantitative way.
Launch OF a rOcket
A rocket’s payload (everything EXCEPT fuel) has a mass of 10,000 kg and ĈV 5 2.5 kJ/kg · K.
Initially, the rocket is at rest at ground level, is at ambient temperature (T 5 258C) and
pressure (P 5 1 bar), and contains the payload plus 100,000 kg of rocket fuel. The fuel
has a specific enthalpy of formation Ĥ 5 1000 kJ/kg, using the same reference state that
is used for the data in Appendix C.
The fuel burns to completion, and the emitted exhaust consists of 30,000 kg water
vapor and 70,000 kg carbon dioxide. The exhaust leaves at T 5 258C and P 5 1 bar and
has a velocity of 3 km/s. When the last of the fuel is consumed, the rocket is 5 km above
the Earth, and the payload has T 5 2508C. What is its velocity at this point? Exhaust is
released continuously as the rocket climbs, but a large fraction of the fuel burns close
to the ground. Assume the average height at which the exhaust is released is 0.5 km.
Assume further that the rocket is adiabatic and that it neither produces nor uses shaft
work.
SOLUTION:
Step 1 Define a system
The rocket, and everything inside it, will be the system.
Step 2 Apply and simplify the energy balance
This example describes the initial and final states of the rocket, but doesn’t indicate the time
required to climb to this height, so we use the time-independent form of the energy balance.
exaMPLe 3-13
Rocket fuel is here
modeled as if it were
a single compound.
Real rocket fuel
would more likely be
a mixture of one or
more fuels plus an
oxidizer to support
their combustion.
FOOd FOr
thOught
3-18
Is it plausible that
the rocket itself is
at 2508C but the fuel
exhaust is at 258C?
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118 Fundamentals of Chemical Engineering Thermodynamics
This is the only
example in the
first eight chapters
of the book that
involves mixtures of
gases or liquids: the
exhaust is a mixture
of CO2 and H2O. A
complete treatment
of the mathematical
modeling of mixtures
begins in Chapter 9.
FOOd FOr
thOught
3-19
If the masses of exiting
water vapor and
carbon dioxide were
not given, how could
you estimate them?
No immediate simplification of the accumulation term is possible; the system experiences significant changes in mass, internal energy, kinetic energy and potential energy.
On the right-hand side, there is no Q or WS. We assume there is also no WEC , because the
size of the rocket doesn’t change. There is no matter entering the system, but the exhaust
gases leave. The energy balance simplifies to
5 1
v2
1 gh
2
D M Û 1
5 1
v2
D M Û 1
1 gh
2
In Chapter 9, the
properties of a mixture
for ideal gases are
found simply by
summing the properties
of the individual pure
gases that make up the
mixture multiplied by
how much of each gas
is present.
1
k51
26
v 2k
1 ghk
2
(3.110)
v 2CO
26 5 2m 1Ĥ 1 2 1 gh 2
2
CO2
CO2,out
1
2
(3.111)
CO2
2 mH O,out ĤH O 1
v 2H O
2
2
1 ghH O
2
2
2
Writing out the initial and final energy of the system explicitly gives
5M 1Û 1 v2 1 gh26 2 5M 1Û 1 v2 1 gh26
2
2
final
1
2
Don’t confuse the
different v terms and
h terms. The left-hand
side represents the
system (rocket) and
the right-hand side
represents material
(exhaust gases)
entering or leaving
the system. Thus, the v
on the left-hand side
is the unknown rocket
velocity, and the v
terms on the righthand side are 3 km/s,
the exhaust velocity.
o5
mk, out Ĥk 1
While the CO2 and H2O leave the system as a single exhaust stream, we haven’t yet
developed how to model mixtures, so we will model them as two separate exiting
streams:
v 2CO
2
5 2mCO , out ĤCO 1
pitFaLL
preventiOn
26
k5K
52
2
2
2
(3.112)
initial
1
1 ghCO 2 mH O, out ĤH O 1
2
2
v 2H O
2
2
2
2
1 ghH O
2
The initial height and velocity are both zero. However, initially, the mass of the system
includes payload and fuel; at the end of the process there is only payload:
5
1
Mpayload Ûpayload 1
1
v2
1 gh
2
5 2mCO , out ĤCO 1
2
2
v 2CO
2 6 2 5M
2
2
payload
6
Ûpayload 1 Mfuel Ûfuel
final
2
1
1 ghCO 2 mH O, out ĤH O 1
2
2
2
v 2H O
2
2
(3.113)
initial
2
1 ghH O
2
Step 3 Insert known values and identify what is unknown
vfinal is the unknown we seek to calculate. Most of the other information in Equation
3.113 is given. The only unknowns we need to resolve are Ûpayload, final and Ûpayload, initial, and
ĤCO and ĤH O.
2
2
Step 4 Collect data
Appendix C contains enthalpy of formation data for CO2 and H2O. Recall that enthalpy is
only known relative to a reference state. The “enthalpy of formation” is the molar enthalpy
relative to a reference state in which all elements have H 5 0 at P 5 1 bar and T 5 25°C. Can
we relate these to ĤCO and ĤH O in Equation 3.113? Let us consider this more closely. The exiting stream is a mixture of carbon dioxide and water vapor. We haven’t yet learned how to calculate Ĥ for a mixture, so we are imagining that there are two separate exiting streams. What
are the pressures of these two streams? Since the total pressure of the exiting gas is P 5 1 atm,
then CO2 and H2O each must have a partial pressure that is less than 1 atm. The value of
H 5 2393.5 kJ/mol in Appendix C is valid for CO2 at P 5 1 bar and T 5 258C; is it applicable
to CO2 at 258C and a lower pressure? We will say “yes” because again, real gases act like ideal
gases at low pressure, and for an ideal gas, enthalpy is not a function of pressure. Converting the
enthalpy of formation data from Appendix C to mass basis using the molecular weight gives
2
2
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C H A P T E R 3 Material and Energy Balances
1 mol2 1 44 g 2 1 kg 2 5 28943 kg
kJ
1 mol 1000 g
kJ
5 213, 430
Ĥ 5 12241.8
2
1
mol
18 g 2 1 kg 2
kg
ĤCO 5 2393.5
kJ
1 mol
1000 g
kJ
(3.114)
2
H2O
Meanwhile, the internal energy of the rocket fuel, Ûfuel, initial, is unknown, but the enthalpy
of formation of the fuel at the correct temperature (25°C) and pressure (P 5 1 bar) is
kJ
given, Ĥfuel, initial 5 1000 . Using the general rule stated in Section 2.3.5 that for liquids
kg
and solids, Û Ĥ. So
kJ
Ûfuel,initial 1000
(3.115)
kg
Step 5 Account for change in internal energy
The left-hand side of Equation 3.114 can be rearranged so that the Ûpayload terms are
grouped together:
d 2 M Û
2 Û
5M 1 2 1 gh 26 1 {M sÛ
v
v
5 2m
1Ĥ 1 2 1 gh 2 2 m 1Ĥ 1 2 1 gh 2
vfinal
2
payload
payload
final
payload,initial
payload,final
2
CO2
CO2,out
CO2
fuel
fuel,initial
H2O
H2O,out
}
(3.116)
H2O
Assuming the rocket is a constant-volume system, the change in internal energy can be
related to ĈV:
Mpayload sÛpayload,final 2 Ûpayload,initial d 5 Mpayload sĈV,payload dsTpayload,final 2 Tpayload,initial d
Using only the
information in the
appendix of this book,
how could you find ĤCO
2
and ĤH O if the exhaust
were at T 5 1008C,
rather than at 258C?
2
pitFaLL
preventiOn
You can’t use the data
in the steam tables
for ĤH O. The data for
rocket fuel, carbon
dioxide, and water
vapor must all have
the same reference
state. The data in
the steam tables and
Appendix C use
different reference
states.
(3.117)
Step 6 Calculate velocity
Inserting Equation 3.117 and all of the known values identified in steps 3 and 4 into Equation 3.116 gives an equation in which the final velocity is unknown. The required algebra
is shown in the more detailed solution given in the supplemental material. The solution is:
vfinal 5 11, 600
FOOd FOr
thOught
3-20
2
2
H2O
CO2
119
m
s
In terms of analyzing a real rocket, Example 3-13 is over-simplified in several ways:
it treated “rocket fuel” as a single compound, assumed the rocket fuel was at ambient conditions initially (real rocket fuel would probably be pressurized), and used an
“average” height at which the fuel was released. A rigorous treatment would include
air resistance, model the rocket’s flight continuously with differential equations, and
combine energy balances with momentum balances.
While simplistic, this example illustrates a system where all three forms of
stored energy—internal, kinetic, and potential—are important and demonstrates
the conversion of chemical energy into kinetic and potential energy. Furthermore, it
illustrates mathematically how a rocket can indeed achieve escape velocity without
violating the first law of thermodynamics.
When ĈV is constant,
the equation
dÛ 5 ĈV dT simply
integrates to
Û2 2 Û1 5 ĈV sT2 2 T1d.
This example is
oversimplified in that
modern rockets are
multi stage; meaning
that portions of the
rocket are fuel tanks
that are jettisoned
when empty. Thus,
Mpayload wouldn’t be
constant.
Recall that the
Motivational Example
stated escape velocity
is ~7 miles/s. Here the
final answer, 11,600 m/s,
is approximately 7.2
miles/s.
3.6 energy balances for common chemical process equipment
The focus of this chapter has been on developing a systematic approach to writing
and solving energy balances that is applicable to any physical system. However, some
specific cases that are of particular interest to chemical engineers are explored here.
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120 Fundamentals of Chemical Engineering Thermodynamics
tabLe 3-1 Steady state energy balance equations for common chemical processes.
unit
Operation
symbols
typical steady state
energy balance
Valve
v 2out
2
Pump, Turbine
or Compressor
Heat
Exchanger
(single stream)
Heat
Exchanger
(both hot and
cold streams)
In Example 3-6 we
defined a system that
included a turbine, but
was larger than just
the turbine, so it would
have been wrong to
apply the turbine
equation from Table
3-1 to that system.
Adiabatic, potential, and
kinetic energy negligible
Ĥin 5 Ĥout
Nozzle
Q
assumptions/simplifications
in energy balance
5 Ĥin 2 Ĥout
Adiabatic and potential
energy negligible, velocity of
entering stream negligible
·
WS
· 5 Ĥout 2 Ĥin
m
Adiabatic, potential, and
kinetic energy negligible
·
Q
5 Ĥout 2 Ĥin
·
m
Potential and kinetic energy
negligible
0
· sĤ 2 Ĥ d
5m
1
1, in
1, out
·
1 m sĤ 2 Ĥ d
Potential and kinetic energy
negligible, no heat lost to
surroundings
2
2, in
2, out
Most large-scale chemical processes are designed to operate at steady state.
Table 3-1 summarizes steady-state energy balance equations for several unit operations commonly found in chemical processes. In all cases, the single piece of equipment
is the system.
The equations in Table 3-1 are broadly useful, but care must be taken to apply them
only to steady-state processes in which the noted assumptions and simplifications are
valid. Notice there is no WEC in any of the equations, but the words “expansion work
negligible” don’t even appear in the “Assumptions/Simplifications” column of the table.
If a process is operating at steady state, the volume of the system cannot be changing, so
no expansion/contraction work can occur. Thus, once we’ve established that a process
is steady state, “expansion work negligible” is not an assumption; it’s inherently true.
Reactors and separators are arguably the most fundamental unit operations in
chemical engineering. These, however, are not included in Table 3-1 for two reasons.
1. Reactors and separators both inherently involve mixtures of compounds. Thus,
while Example 3-13 did involve a chemical reaction, we defer any in-depth discussion of reactions and separations until the second half of the book. In Chapters 9–13, when we develop methods of modeling the thermodynamic properties
of mixtures, many of the examples examine the thermodynamics governing separation processes. Chapter 14 covers chemical reactions.
2. There are many different types of reactors and separators; it isn’t realistic to propose a single “typical energy balance equation” for these operations.
The following sections discuss the unit operations summarized in Table 3-1.
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C H A P T E R 3 Material and Energy Balances
3.6.1 valves
AlexKZ / Shutterstock.com
Paul Broadbent / Shutterstock.com
Valves can be used to control the flow rate or pressure of a stream. Some pictures of
common types of valves are given in Figure 3-18.
The primary physical effect of a valve on the fluid flowing through it is a decrease
in pressure. This decrease is a result of the constriction in diameter.
Figure 3-18 Photos of valves.
While it takes work to open and close a valve, the opening or closing is a transient
process. During steady-state operation we expect the state of the valve (i.e., how far
open) to be static. Consequently, there is no mechanism for transfer of shaft work to
or from the fluid. Valves are commonly assumed adiabatic: if the contents of a pipe are
at high temperature, one might need to account for heat lost in a long section of pipe,
but can safely ignore heat lost in the small space occupied by the valve. Thus, if potential and kinetic energy are considered negligible, the energy balance simplifies to:
· Ĥ 2 m
· Ĥ
05m
(3.118)
in
in
out
out
But because the entering and leaving mass flow rates are equal at steady state, the
mass flow rates can be divided out, providing the energy balance shown in Table 3-1:
0 5 Ĥin 2 Ĥout
or equivalently
Ĥin 5 Ĥout
(3.119)
3.6.2 nozzles
A nozzle is designed to accelerate a fluid to high velocity, converting some of the
fluid’s internal energy into kinetic energy. Rockets operate on the principle of conservation of momentum; the exhaust is expelled through nozzles, giving it a very
high downward velocity, which results in an upward velocity for the rocket. A more
familiar everyday example of nozzles is an attachment on a garden hose. In principle
the water leaving the nozzle must be slightly colder than the water in the hose, but
the velocities achieved by these nozzles are modest enough that the effect likely isn’t
noticeable to your bare hands.
The typical energy balance for a nozzle was derived in the context of Example 3-4.
3.6.3 Pumps, compressors, and turbines
While pumps, compressors, and turbines are distinct unit operations, they are here
discussed simultaneously because in steady-state operation they typically have identical energy balance equations, and there is also some overlap in the physical phenomena at work in these three processes.
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121
122 Fundamentals of Chemical Engineering Thermodynamics
Turbines were discussed in Section 1.2.3 and Example 3-6. Their purpose (in
effect) is to convert internal energy into shaft work. A turbine can be pictured as
two steps: (1) a nozzle is used to accelerate the fluid and (2) the fluid moves over the
blades of a windmill. The nozzle converts internal energy into kinetic energy, and
the windmill converts kinetic energy into rotary motion or shaft work. The turbine
encompasses both steps. The fluid leaving the nozzle is at high velocity, but this
fluid never crosses the boundaries of the system. This allows us to consider kinetic
energy as negligible in the energy balance around a turbine; the fluid entering the
nozzle and the fluid leaving the windmill are both moving at comparatively low
velocity.
Turbines normally are modeled as adiabatic, meaning that the only significant
terms of the energy balance are the enthalpy of the entering and leaving streams and
the shaft work produced.
· Ĥ 2 m
· Ĥ 1 W·
05m
(3.120)
in
out
out
S
Jan de Wild / Shutterstock.com
The energy balance
derived here for a
turbine is identical
to that derived in
Example 3-8 for
a pump.
in
The mass flow rates entering and leaving are equal, so the “in” and “out” subscripts
can be eliminated. It is convenient to rearrange the equation so that it is explicit in
“work produced per mass of fluid”:
·
WS
(3.121)
· 5 Ĥout 2 Ĥin
m
The turbine always produces a substantial drop in pressure. The converse unit operations are the pump and compressor, both of which are designed to increase the pressure of a fluid, which is accomplished through the addition of work. While the intent
of pumps and compressors are identical, the physical equipment is very different, as
illustrated in Figure 3-19. Pumps are used to compress liquids and compressors are
used for gases. Both pumps and compressors can be damaged if liquid-vapor mixtures are fed into them.
Figure 3‑19 Photo of a compressor.
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C H A P T E R 3 Material and Energy Balances
123
Since pumps, compressors, and turbines are all well modeled as adiabatic, the
typical steady-state energy balance equations for pumps and compressors are identical to that for turbines. For pumps and compressors, work should be positive (energy
is added to the system in the form of work, Ĥout . Ĥin) while for turbines, work is
negative (Ĥout , Ĥin, energy is transferred to the surroundings.) For our purposes,
applying the energy balance is typically the only way to determine the shaft work
for a turbine or compressor. For a pump, however, the shaft work can frequently be
estimated using Equation 3.78:
·
WS 5
#
P 5 Pout
·
VdP
P 5 Pin
with the assumption that volume is constant for the liquid.
3.6.4 heat exchangers
Heat exchangers are unit operations intended to add or remove heat from streams;
among the many applications are changing the temperature of the stream to a desired value or changing the phase of the stream (as in the condenser and boiler in the
Rankine cycle described in Section 2-1).
The classical design for a heat exchanger is the shell-and-tube heat exchanger
shown in Figure 3-20. A large number of small tubes provide more surface area than
a single large tube, thereby allowing more efficient heat transfer. One fluid (the
“tube-side” fluid) travels through the narrow tubes, and the other phase (the “shellside” fluid) surrounds the tubes. Thus, heat is exchanged between the two fluids without the fluids coming into direct contact with each other.
Heat exchangers are
frequently abbreviated
HE or HX in drawings
and schematics.
FOOd FOr
thOught
3-21
Is the bulb of a
mercury thermometer
a heat exchanger?
Cold fluid
Shell
Header
Tubes
Hot
fluid
Cooled fluid
Courtesy of Allteko.
Heated fluid
Figure 3‑20 Schematic and photos of shell-and-tube heat exchanger.
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124 Fundamentals of Chemical Engineering Thermodynamics
Shell side
Tube side
Shell side
Shell side
Shell side
Tube side
Tube side
Shell side
Shell side
(a)
(b)
Figure 3‑21 Possible system definitions: (a) the entire heat exchanger, (b) one side of the heat
exchanger.
If the system is an entire shell-and-tube heat exchanger, the heat transfer occurs
inside the boundaries of the system, as illustrated in Figure 3-21a. Thus, assuming no
heat is lost to the surroundings, the Q and W terms of the energy balance are zero.
However, in this case there are two streams entering and two streams leaving. Using
the subscripts 1 and 2 to distinguish the two fluids from each other gives
· sĤ 2 Ĥ d 1 m
· sĤ 2 Ĥ d
05m
(3.122)
1
1,in
1,out
2
2,in
2,out
While Equation 3.122 is a correct energy balance for complete shell-and-tube
type heat exchangers, one could also define one side or the other of the heat exchanger as the system, as illustrated in Figure 3-21b. Here, there is only one material stream entering and leaving the system, and Q is not equal to zero; the heat
transfer does cross the boundaries of the system when the system is defined this
way. The energy balance for “one side” of a heat exchanger, rearranged to mirror
Equation 3.121, is thus:
·
Q
(3.123)
· 5 Ĥout 2 Ĥin
m
There are several reasons why one might use Equation 3.123 rather than Equation 3.122. Two examples are given here.
Some heat exchangers don’t have two distinct fluids. If the heat is supplied by
electricity, then the heat exchanger has material flow on only one “side”; Equation 3.122 would not apply.
In a design setting, especially early in the design process, one frequently wants
to know how much heat would be required to accomplish a specific task, but
doesn’t yet know what the source of the heat will be. Indeed, the decision
regarding the source of the heat might itself be dependent upon the value
of Q!
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C H A P T E R 3 Material and Energy Balances
125
3.7 s u M M a r y O F c h a P t e r t h r e e
Mass and energy can both be treated as conserved quantities; they cannot be
created or destroyed.
The principles of mass conservation and energy conservation can be expressed
mathematically through material balances (Equation 3.1) and energy balances
(Equation 3.34).
The energy balance equation is a mathematical statement of the first law of
thermodynamics.
The generalized material and energy balances can be applied to any clearly
defined system.
Any more simplified version of a material or energy balance, such as the ones in
Table 3-1, can only be applied when the assumptions used to derive the equation
are understood and are valid for the system of interest.
A process that has a known initial state and a known final state, but an unknown
duration, can be analyzed using time-independent material and energy balances
(Equations 3.31 and 3.35).
Material and energy balances both take the form accumulation 5 in 2 out.
For a steady-state process, accumulation 5 0.
Kinetic and potential energy are often negligible in typical chemical processes.
3.8 e x e r c i s e s
3-1.
3-2.
3-3.
3-4.
10 m3 of saturated steam at T 5 1508C is mixed
with 0.1 m3 of saturated liquid water at T 5 1508C.
How many total kilograms of H2O does the mixture
contain?
A 1 m3 vessel contains a mixture of saturated liquid
water and saturated steam at T 5 2008C. If the vessel
contains 15 kg in total, find the mass of liquid water
and the mass of steam.
Two moles of an ideal gas with CV* 5 3R are confined
in a piston-cylinder arrangement. The piston is frictionless and the cylinder contains no mechanism for
shaft work. Initially, the temperature is 300 K and the
pressure is 1 bar. The external atmospheric pressure
is also 1 bar. Find the heat, work and change in internal energy of the gas if:
A. It is heated to 500 K at constant volume.
B. It is heated at constant pressure to 500 K.
C. It is compressed isothermally to 5 bar.
A mole of gas in a closed system undergoes a fourstep, cyclic process that returns it to the original state 1.
The following table gives some data on the steps of
the process. Fill in the blanks.
Step
∆U (J)
1�2
2200
3-5.
26000
2800
3�4
1 �2�3�4�1
W(J)
23800
2�3
4�1
Q (J)
300
4700
21400
1.5 kg of water is placed in a closed container. The
vessel has a frictionless piston that maintains a constant pressure. This water is initially all saturated liquid at 808C, and is boiled into saturated vapor. The
pressure opposing expansion can be assumed equal
to the pressure inside the cylinder.
A. At what pressure does the water boil?
B. What are the initial and final volumes of the
cylinder?
C. What is the work done in this process?
D. What is the change in internal energy of the
water?
E. What is the heat added for this process?
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126 Fundamentals of Chemical Engineering Thermodynamics
3-6.
Liquid water enters a steady-state heat exchanger
with a flow rate of 200 kg/min at 15 MPa and 3358C.
It exits the boiler as superheated steam at 15 MPa
and 6008C. There is no mechanism for shaft work in
the heat exchanger.
A. Find the difference in specific enthalpy between
the entering and exiting streams.
B. Find the rate at which heat is added to the steam
in the heat exchanger.
3-7.
Steam enters an adiabatic, steady-state turbine at
T 5 3008C and P 5 5 bar. The effluent from the turbine has P 5 1 bar, and is a liquid–vapor mixture
with q 5 0.9.
A. Find the difference between Ĥ of the exiting
stream and Ĥ of the entering stream.
B. In order to produce 100 MJ/min of work, what is
the required flow rate of the entering stream?
3.9 P r O b L e M s
3-8.
3-9.
A swimming pool is 12 feet wide, 30 feet long, and
10 feet deep, and the pool is initially 90% filled with
water. A hose is placed in the pool, and water flows
into the pool through the hose at a constant rate of
one lbm every 8 seconds, until the pool is completely
filled. During this time, an inch of rain falls, and a
30 gallon rain barrel full of water is emptied into the
pool. How long, from the moment the hose is turned
on, does it take to fill the pool? Assume the density
of water is constant at 62.3 lbm/ft3.
A 4 m3 storage tank is filled with steam and maintained at a constant T 5 2508C. The initial pressure
is 20 bar. A valve is opened and 1 kg/min of steam
leaves the tank.
A. What is the initial mass of the steam in the tank?
B. How much time does it take for the pressure to
decrease to 5 bar?
3-10. Many chemical products are synthesized by biological mechanisms, in which living cells take in
“reactants” as food, and emit “products.” A typical fermentation process involves at least two
stages— in the first, cells multiply until the desired concentration of cell mass is reached, and
in the second, the bulk of the product is synthesized. A cell is a complex living organism, not a
single chemical compound. However, in modeling
fermentation processes, it is sometimes useful to
assign an empirical chemical formula to the cells.
Here we will assume the cells, or the “biomass,”
can be represented by the chemical formula
C4H7O2N.
The “growth phase” of a fermentation process
produces a broth that has a volume of 5000 L and
contains 100 g/L of biomass.
A. If all of the nitrogen in the biomass comes from
ammonia, what mass of ammonia was required to
produce the biomass?
B. If all of the carbon in the biomass comes from
glucose (C6H12O6), what mass of glucose was required to produce the biomass?
C. If the ammonia from part A and the glucose from
part B are added to a batch fermentation reactor,
and are completely consumed forming biomass,
how much by-product is formed? Can you propose a balanced chemical reaction that represents
the process?
3-11. A liquid enters a steady-state flash chamber with a
flow rate of 100 mol/min. One liquid stream and one
vapor stream, each at T 5 508C and P 5 1 bar, exit.
The exiting vapor is an ideal gas with a flow rate of
1.8 m3/min. What are the molar flow rates of the liquid and vapor product streams?
3-12. Water in a creek is at T 5 258C and is flowing at 2 m/s
when it reaches the top of a waterfall. It falls 50 meters into a pool. The velocity leaving the pool is negligible. The creek, waterfall, and pool can be modeled
as at steady state and adiabatic.
A. What is the velocity of the water at the point that
it hits the surface of the pool?
B. Assuming the heat capacity of liquid water is constant at 4.19 kJ/kg⋅8C, and neglecting any external
heat source, what is the temperature of the water
in the pool?
3-13. One kilogram of liquid water is placed in a pistoncylinder device, initially at P 5 1 bar and T 5 58C. At
these conditions, the density of water is 1000.4 g/L.
The water is heated at constant pressure to T 5 958C,
at which point the density is 962.3 g/L. The external
pressure is also P 5 1 bar.
A. Find the work and heat for this process. Use the
temperature-dependent value of CP given in
Appendix D. Apply the relationship H 5 U + PV
if conversion between H and U is necessary.
B. Find the heat for this process. This time, assume
the density is constant at 1000 g/L, that CP is constant at 4.19 J/g · K, that CV ~ CP, and that U ~ H
for a liquid.
C. Compare your answers to parts A and B. The simplifying approximations in part B are often used for
liquids (and solids); how well do they work here?
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C H A P T E R 3 Material and Energy Balances
127
3-14. One kilogram of steam is placed in a piston-cylinder
device, initially at P 5 1 bar and T 5 1008C. The
steam is heated at constant pressure to T 5 2008C.
A. Find the heat and work for this process. Obtain all
needed data from the steam tables.
B. Find the heat and work for this process. Use the
ideal gas law and the ideal gas heat capacity for
steam, located in Appendix D. How far off are
your answers compared to part A?
C. Suppose in part A you had overlooked the fact
that the steam expands when heated, and assumed
that work was negligible. How far off would your
answers for Q have been?
model the evaporating water as saturated vapor
at T 5 20°C or saturated vapor at P 5 1 bar.
Which is a better estimate?
C. If 200 kg of water evaporated, and at the end of
the day, the temperature of the lake is again a uniform 20°C, calculate the NET amount of heat that
was added to or removed from the lake.
D. Calculate the NET heat if the final temperature
was a uniform 21°C and the amount of water
evaporating was 200 kg.
E. Calculate the NET heat if the final temperature
was a uniform 21°C and the amount of water
evaporating was 50 kg.
3-15. An adiabatic valve operates at steady state. Saturated liquid water at P 5 5 bar enters. The exiting
stream is a saturated liquid–vapor mixture at P51
bar. What is the quality of the exiting stream?
3-18. 10 mol/s of gas enters a steady state, adiabatic nozzle
at T 5 3008C and P 5 5 bar and leaves at T51008C
and P 5 1 bar. Find the exiting velocity of the gas if it is:
A. Steam
B. Nitrogen, modeled as an ideal gas
C. An ideal gas with CP 5 (7/2)R and a molecular
mass of 28 g/mol
3-16. In a large chemical plant, steam is used as a heat
source in several different processes. The steam
condenses in heat exchangers, and the liquid water
is recycled to a boiler, which converts it back into
steam. Because the liquid water streams are coming
from different processes, they are at different conditions, but there is one uniform exit stream which is at
P 5 8 bar.
The boiler operates at steady state and has the
following streams entering:
100 kg/min of saturated liquid at T 5 1508C.
200 kg/min of saturated liquid at T 5 1808C.
75 kg/min of saturated liquid at P 5 6 bar.
A. What rate of heat must be added to the boiler if
the exiting steam is to be at T 5 3008C?
B. If the rate at which heat is added is 800,000 kJ/min,
what is the temperature of the exiting steam?
3-17. A lake initially contains 20,000 kg of water and has a
uniform temperature of 20°C. The ambient pressure
is P 5 1 bar. During the course of a day, the following
events affecting the lake occur:
200 kg of rain falls into the lake. The rainwater has
a temperature of 25°C.
Some water evaporates from the lake.
Heat is both added to (sunlight) and removed
from (convection into the cooler air) the lake.
If you need the heat capacity of LIQUID water, use
the approximation CV ~ CP ~ 4.184 J/g · 8C.
A. Find a reasonable estimate of the velocity of a
raindrop and determine the kinetic energy of the
rain when it strikes the lake. Is the kinetic energy
significant compared to the enthalpy?
B. The surface of the lake is at T 5 20°C and P 5 1
bar. The enthalpy of water vapor at T 5 20°C and
P 5 1 bar is not in the steam tables. You could
3-19. This problem examines the effect of mixing water
at two different temperatures and/or two different
phases, such as dropping ice cubes into warm water.
Assume that DĤfus 5 333.55 J/g, that CP for ice is constant at 2.11 J/g · K, and that CP for liquid water is
constant at 4.19 J/g · K. For each the following cases,
assume the mixing is adiabatic and carried out at
atmospheric pressure. For each case, determine the
temperature and phase of the water when the mixing
is complete. If there are two phases in equilibrium,
determine the mass of each phase.
A. 100 g of liquid water at 508C is mixed with 50 g of
liquid water at 08C.
B. 100 g of liquid water at 508C is mixed with 50 g of
ice at 08C.
C. 50 g of liquid water at 08C is mixed with 50 g of ice
at 08C.
D. 50 g of liquid water at 208C is mixed with 50 g of
ice at –208C.
3-20. Superheated steam enters a nozzle that has an inlet
diameter that measures 2.5 cm and an outlet diameter that measures 1 cm. Its outlet velocity has been
measured at 30 m/s. The steam that enters the nozzle
is at 6508C and 1350 kPa and the steam that exits the
nozzle is at 4008C and 100 kPa.
A. Determine the velocity of the steam upon entering the nozzle.
B. Determine the rate at which this process gains or
loses heat.
C. Example 3-4 and Section 3.6.2 indicate that in
modeling a nozzle, it is common to assume the
nozzle is adiabatic and that the velocity entering
the nozzle is negligible. Do your answers in this
case seem consistent or inconsistent with those
common assumptions?
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128 Fundamentals of Chemical Engineering Thermodynamics
3-21. A rigid, 10 m3 vessel initially contains 100 kg of water/
steam at P 5 2 bar. 10 kilograms of steam at P 5 5
bar and T 5 3008C is gradually added to the vessel.
During the same time period, heat exchange occurs
with the surroundings. The final water/steam mixture
is at P 5 3 bar.
A. What fraction of the initial contents of the vessel
were in the vapor phase?
B. What fraction of the final contents of the vessel
are in the vapor phase?
C. How much heat was added or removed?
3-22. A vessel initially contains 1000 kg of saturated liquid
water at T 5 308C, which needs to be cooled. The vessel is adiabatic but has a piston that allows pressure
and volume to be changed and a valve that can be
opened to allow vapor to escape through the top. So,
a small portion of the water is allowed to evaporate
and escape, resulting in the cooling of the remaining
water. Calculate each of the following, and state and
justify any assumptions that you make.
A. The temperature of the water remaining in the
vessel if 1 kg of water evaporated.
B. The amount of water that would have to evaporate in order to cool the water remaining in the
vessel to 258C.
C. The amount of water that would have to evaporate in order to cool the water remaining in the
vessel to 208C.
3-23. Water at P 5 25 bar and T 5 2008C enters a steadystate flash chamber with a flow rate of 5 kg/s. The liquid and vapor streams exiting the flash chamber are
both at P 5 1 bar.
A. If the flash chamber is adiabatic, what fraction of
the entering water leaves the flash as vapor?
B. How much heat must be added to or removed
from the flash chamber in order to vaporize half
of the entering water?
3-24. This problem presents a comparison of the work required in adiabatic pumps and compressors.
A. Saturated water vapor enters a steady-state compressor at P 5 1 bar. Estimate the work required
to compress this vapor up to P 5 10 bar and
T 5 4008C.
B. Saturated liquid water at P 5 1 bar enters a pump.
Estimate the work required to pump the liquid up
to P 5 10 bar, and estimate the temperature of the
exiting liquid.
3-25. A two-step, steady-state process is used to compress
100 kg/min of nitrogen from P 5 1 bar and T 5 250 K
to P 5 10 bar and T 5 250 K. First, an adiabatic
compressor is used to convert the nitrogen from
P 5 1 bar and T 5 250 K to P 5 10 bar and
T 5 300 K. Then a heat exchanger is used to cool
the nitrogen to T 5 250 K. Find the work added in
the compressor, and the heat removed in the heat
exchanger.
A. Use an ideal gas model.
B. Use Figure 2-3 as much as possible. If you need
an equation of state, use the van der Waals
equation, with a 5 1.37 3 106 bar · cm6/mol2 and
b 5 38.6 cm3/mol.
3-26. 1 kg of nitrogen is contained in a piston-cylinder device. The nitrogen is isothermally compressed from
P 5 1 bar to P 5 10 bar at T 5 300 K. Find the initial
volume, final volume, and work and heat added to or
removed from the nitrogen.
A. Assume nitrogen behaves as an ideal gas.
B. Use Figure 2-3 as much as possible. If you need
an equation of state, use the van der Waals equation, with a 5 1.37 3 106 bar · cm6/mol2 and b 5
38.6 cm3/mol.
Problems 3-27 through 3-29 involve the same compound that was considered in Chapter 2, Problems
2-22 through 2-26:
A molecular weight of 50 lb/lb-mol
A normal melting point of 508F
A normal boiling point of 1508F
A constant heat capacity in the solid phase of
CP 5 0.7 BTU/lb · 8R
A constant heat capacity in the liquid phase of
CP 5 0.85 BTU/lb · 8R
A constant density in the solid phase of r 5 40 lb/ft3
A constant density in the liquid phase of r 5 30 lb/ft3
An ideal gas heat capacity of CP* 5 1 2 T/1200 1
T 2 3 1026 BTU/lb · 8R, where T is the temperature
in 8R
An enthalpy of fusion DĤfus 5 75 BTU/lb at atmospheric pressure
An enthalpy of vaporization DHˆ vap 5 125 BTU/lb
at atmospheric pressure
3-27. 20 lb-mol/min of the compound enters a steady-state
boiler as saturated liquid at P 5 1 bar. Find the rate
at which heat is added if the exiting stream is:
A. Saturated vapor at P 5 1 atm
B. Vapor at P 5 1 atm and T 5 2008F
C. Vapor at P 5 0.7 atm and T 5 2008F
3-28. Initially, one lb-mol of the compound is placed in a
piston-cylinder device at T 5 258F and P 5 1 atm.
The compound is heated at constant pressure. Find
the heat (Q) added if the final state is:
A. 50% solid and 50% liquid at T 5 508F
B. Liquid at 1258F
C. 25% vapor and 75% liquid at T 5 1508F
D. Vapor at 2008F
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C H A P T E R 3 Material and Energy Balances
129
saturated vapor at P 5 1.1 bar; the rest is removed
as the bottom product.
3-29. A liquid stream contains 1 lbm/s of the compound at
T 5 1008F and P 5 1 atm. It needs to be boiled and
heated to P 5 1 atm and 1758F, as that is the temperature at which it must enter a chemical reactor. This
heating/boiling process is to be carried out in a shelland-tube heat exchanger with steam at P 5 1 bar and
T 5 3008C to be used as the source of the heat. Find
the flow rate of entering steam required if
A. The steam exits the heat exchanger as saturated
vapor at P 5 1 bar.
B. The steam condenses completely, and exits the
heat exchanger at saturated liquid at P 5 1 bar.
8 mol/s of benzene enters the condenser as saturated vapor at P 5 0.9 bar. All of the entering vapor
is condensed into saturated liquid at P 5 0.9 bar.
75% of the liquid benzene is returned to the column and the rest is removed as the top product.
The heat for the reboiler is to be provided by saturated steam, which is available at P 5 1 bar, P 5 3
bar, or P 5 5 bar.
Either air or water can be used as the coolant in
the condenser. Either way, the coolant enters at
P 5 1 bar and T 5 258C, and leaves at P 5 1 bar
and a temperature 108C lower than the temperature of the benzene in the condenser.
3-30. A small water tower has a cylindrical chamber 3 m
in diameter and 4 m tall. The bottom of the cylinder
is 15 m above the ground. Water is pumped from an
underground reservoir into the chamber, via a pipe
that enters through the bottom of the chamber. The
surface of the reservoir is 5 m below the ground. Assume that the tower, reservoir, and pipes are adiabatic, and that all water is at a uniform temperature,
and a uniform density of 1000 g/cm3.
A. When the water tower is completely empty, what
is the minimum pump work required to move 1 kg
of water into the tower?
B. When the water tower is almost full, what is the
minimum pump work required to move 1 kg of
water into the tower?
C. A home has a second floor faucet is 3 m above the
ground. The ground in the area is level, and there
is nothing powering the water except the force of
gravity. When the faucet it turned on, what is the
maximum velocity the water can have?
D. Suppose the water tower has these same dimensions but is located on top of a hill. Repeat part C,
assuming the altitude of the water faucet is 5 m
below the altitude at the top of the hill.
You will probably have to look beyond this book
to find physical properties of benzene, toluene, and
water that are needed to solve the problem. Indicate your sources. If you can’t find the exact data
you need, state and justify any assumptions that you
make in solving the problem.
A. Determine the rate at which heat is added to the
toluene in the reboiler.
B. Determine the flow rate of steam entering the
reboiler, for each of the three possible inlet pressures. In each case, assume the steam is completely condensed and leaves the reboiler as
saturated liquid, with the same pressure at which
it entered.
C. Comment on the results of part B. Are any of the
three steam pressures obviously better or worse
options than the others? Knowing that the higherpressure steam is more expensive than the lowerpressure steam, is there any rationale for using the
higher-pressure steam?
D. Determine the flow rate of water, if water is used
as the coolant.
E. Determine the flow rate of air, if air is used as the
coolant. Assume air at P 5 1 bar acts like an ideal
*
*
*
gas, and that C P,air
5 0.79 CP,N
1 0.21 CP,O
2
2
F. The air for the condenser is at the local ambient
conditions and can be considered free. The water
for the condenser is not free. Do you see any
rationale for using water instead of air?
3-31. A steady-state distillation column is designed to separate benzene from toluene. The separation is nearly
enough complete that, for the purposes of designing
the reboiler and the condenser, we can model the
material in the reboiler as pure toluene and the material in the condenser as pure benzene.
10 mol/s of toluene enters the reboiler as saturated liquid at P 5 1.1 bar. 80% of the entering
toluene is boiled and returned to the column as
3.10 g LO s s a r y O F s yM b O L s
A
area
CV
constant-volume heat capacity (molar basis)
C
number of chemical compounds
CV*
constant-volume heat capacity (molar basis) for
ideal gas
ĈV
constant-volume heat capacity (mass basis)
CP
constant-pressure heat capacity (molar basis)
CP*
constant-pressure heat capacity (molar basis) for
ideal gas
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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130 Fundamentals of Chemical Engineering Thermodynamics
n
number of moles added to or
removed from system
v
velocity
n·
V
volume
degrees of freedom
molar flow rate
V̂
specific volume
g
gravitational constant
P
pressure
V
molar volume
H
enthalpy
potential energy
shaft work
specific enthalpy
rate of heat addition
WS
Ĥ
P.E.
·
Q
H
molar enthalpy
Q
heat
rate of shaft work
h
height
R
gas constant
WEC
·
WS
·
WEC
K.E.
kinetic energy
r
radius
M
mass of system
T
temperature
ΔH 0f
enthalpy of formation
m
mass added to or removed
from system
t
time
r
density
U
internal energy
π
number of distinct phases
·
m
mass flow rate
Û
specific internal energy
N
number of moles in system
U
molar internal energy
ĈP
constant-pressure heat
capacity (mass basis)
F
expansion/contraction work
rate of expansion/contraction
work
3.11 reFerences
A. C. Clarke, Profiles of the Future, Harper & Row, New
York, 1962.
J. Kluger, “Rocket Scientist: Robert Goddard,” Time, 153,
12, March 29, 1999.
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Entropy
4
Learning Objectives
This chapter is intended to help you learn how to:
Define a reversible process and recognize when processes are reasonably modeled as
reversible
Define entropy and predict qualitatively whether a physical change (melting, boiling,
temperature increase, volume increase, etc.) should cause entropy to increase or
decrease
Recognize the irreversible features of real processes that lead to generation of entropy
Quantify changes in the entropy of a substance using the fundamental definition of
entropy
Write entropy balances that describe a variety of physical systems
Compute the maximum work that can be produced by a process, or the minimum work
required to accomplish a specific task
Use efficiencies to compare real processes to idealized processes
Apply entropy balances in combination with energy balances to solve engineering
problems
T
he conservation of mass and energy are foundational engineering principles that
can be applied to a vast array of engineering problems. Conversion of energy from
one form to another is a common theme in engineering applications throughout the
first three chapters, and the generalized energy balance is a powerful fundamental tool in
modeling these applications. However, by itself, the energy balance might imply that it is
possible to convert energy from any form into any other, at any time, without restrictions
or limitations of any kind. This is not true, and one role of the concept of entropy in thermodynamics is revealing the limitations on energy conversion. This chapter examines entropy
and its use.
FOOd FOr
thOught
4-1
Suppose ice cubes
were placed in a warm
glass of water, and the
ice cubes got colder
while the water got
warmer. Is the first law
of thermodynamics
violated?
4.1 MOtivatiOnaL exaMPLe: turbines
This chapter introduces a new state property called entropy. We first illustrate how
entropy is a useful property through a closer examination of turbines. The introductions to Chapters 1 and 2 discussed the Rankine cycle, how the turbine is the unit
operation in the cycle that actually produces work, and how the fluid experiences
131
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132 Fundamentals of Chemical Engineering Thermodynamics
large pressure drops in the turbine. But in previous examples, the inlet and outlet
conditions of the steam were simply given. How can we relate the work produced to
the pressure drop in a more fundamental way? We will see that this is a question for
which the energy balance, while useful, does not tell the whole story.
examPle 4‑1
FOur POssibLe turbine cOnFiguratiOns
Steam at 550°C and 30 bar enters a steady-state, adiabatic turbine (Figure 4-1). The
exiting stream is at P 5 1 bar. Find the work produced per kilogram of entering steam if
the exit stream is
A. Saturated liquid
B. Saturated vapor
C. Superheated steam at 200°C
D. A 50-50 mixture (by mass) of saturated liquid and saturated vapor
Steam
P 5 30 bar
T 5 5508C
P 5 1 bar
T5?
Ws
Figure 4‑1 Turbine described in Example 4-1.
SOLUTION:
·
WS is the rate at which
work is added. The
·
time dimensions of WS
·
and m cancel, giving
dimensions of energy
per unit mass.
Step 1 Define the system and write the energy balance
In Section 3.6.3, we applied the energy balance to a steady-state, adiabatic turbine. Using
the turbine as the system, the energy balance simplifies to:
·
WS
· 5 Ĥout 2 Ĥin
m
The left-hand side is exactly what we’re trying to find: work per unit mass of entering
steam.
Step 2 Collect data
The right-hand side can be evaluated using the steam tables. The entering stream is the
same in all four variations of the problem:
Ĥin 5 3569.7 kJ/kg
(steam at 550°C and 30 bar)
In parts A through C, the exiting stream is a single phase, so Ĥout is also obtained directly
from the steam tables.
A. Ĥout 5 417.5 kJ/kg
(saturated liquid at 1 bar)
B.
Ĥout 5 2674.9 kJ/kg
(saturated vapor at 1 bar)
C.
Ĥout 5 2875.5 kJ/kg
(superheated vapor at 1 bar)
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C H A P T E R 4 Entropy
133
Part D requires an additional step.
Step 3 Compute properties of a 50-50 mixture
In Example 2-1, an intensive property of a system that contains both saturated liquid
and saturated vapor was found as a weighted average of the properties of the individual
phases. For specific enthalpy, the expression is
Ĥ 5 s1 2 qdĤL 1 qĤV
(4.1)
In part D, the exiting stream is a mixture of liquid and vapor, so Ĥout can be found using
equation 4.1. The vapor fraction or quality (q) is given as 0.5 (50-50 mixture), and ĤL and
ĤV are the specific enthalpies of the saturated liquid and vapor at 1 bar, which we found for
parts A and B.
1
Ĥout 5 s1 2 0.5d 417.5
2
1
2
kJ
kJ
kJ
1 s0.5d 2674.9
5 1546.2
kg
kg
kg
(4.2)
Step 4 Plug values into energy balance
The results are summarized as follows:
Part
Ĥin (kj/kg)
Ĥout (kj/kg)
· ·
Ws /m
5 Ĥout 2 Ĥin (kj/kg)
A
3569.7
417.5
23152.2
B
3569.7
2674.9
2894.8
C
3569.7
2875.5
2694.2
D
3569.7
1546.2
22023.5
Example 4-1 proposes four possible exit streams for a turbine. Based on the energy
balance, all of the scenarios presented look plausible and we would clearly say that
example A was the “best”: it produces the most work. But can we simply “choose”
outlet stream A? No, we cannot. Designers do not have infinite freedom to specify
process variables. For example, in practice one normally controls the temperature of
a piece of equipment by adding or removing heat. Consequently, as the designer, if
you define a piece of equipment as adiabatic, you can’t realistically also choose a specific outlet temperature. So when we feed real steam into a real turbine, what actually comes out? The energy balance can be applied to many possible outlet streams,
but it can’t tell us which one is “right.”
In reality, parts A, B, and D in Example 4-1 are all physically impossible. The energy
balance would never have told us this, but the concept of entropy does.
· ·
= 23152.2 kJ/kg.
Looking, for example, at part A, we see that the calculated work is WS /m
And it’s true that 3152.2 kJ of energy must be removed in order to convert one
kilogram of steam at 5508C and 30 bar into one kilogram of saturated liquid water
at P = 1 bar. However, there is simply no way to remove all of this energy in the
form of work. There is a theoretical limit to how much work can be obtained when
the steam expands to a pressure of 1 bar. In Example 4-7, we will use the concept of
entropy to prove that this maximum is 2887.4 kJ for each kilogram of steam. Parts
A, B, and D are physically impossible; the maximum work is exceeded. Part C is
physically possible, but the work obtained is 193.2 kJ/kg less than the maximum; you
might call this 193.2 kJ/kg of “lost work.” In this chapter, entropy allows us to quantify maximum work and lost work.
It might seem strange
that we are talking
about the “maximum”
work being a negative
number. Work
produced by the
system is energy leaving the system, and
therefore negative.
·
The negative WS with
the largest absolute
value corresponds to
the maximum work
produced.
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134 Fundamentals of Chemical Engineering Thermodynamics
In applying the concept of entropy, two points are helpful.
1. Entropy is a state property.
2. The entropy of the universe cannot decrease.
Equation 4.3 quantifies the rate at which
entropy is transported
in a single stream.
Section 4-4 presents
the entropy balance,
which we can use to
prove the italicized
statement in a rigorous mathematical way.
Knowing these two facts actually allows us to show quantitatively that parts A, B,
and D in Example 4-1 are impossible. If we define the turbine as our system, then
the system is at a steady state, meaning that the entropy of the system (like all state
properties) must be constant. Since the entropy of the universe cannot decrease, and
no entropy is accumulating in the system, the rate at which entropy leaves the system
must be greater than or equal to the rate at which entropy enters the system.
We will learn in Section 4.3.1 that heat transfer affects the entropy of a system,
but here the system is adiabatic. The only way entropy enters or leaves this system is
through material streams, quantified by the mass flow rate times the specific entropy:
·
· Ŝ
S5m
(4.3)
The mass flow rates of the entering and exiting streams are equal in this steady-state
process. Consequently, based on the previous paragraph, we can say that the specific
entropy of the stream leaving the turbine can’t be smaller than the specific entropy of
the entering stream.
Because entropy is a state property, there is only one value the specific entropy of
superheated steam at T = 550°C and P = 30 bar can have. That value, Ŝ =7.3768 kJ/kg · K,
is available in the steam tables. According to the steam tables, the entropy of saturated steam at P = 1 bar is 7.3588 kJ/kg · K. The entropies of saturated liquid or of
liquid–vapor mixtures at this pressure are even smaller. All of these values are less
than 7.3768 kJ/kg · K and therefore are unrealistic. However, the specific entropy of
steam at P = 1 bar and T = 200°C is 7.8356 kJ/kg · K; this at least is a physically realistic value based upon what we have said about entropy so far.
At this point, even if we have relatively little insight into what entropy actually
is, we can see that the concept has practical value. The energy balance is very useful
in solving engineering problems, but by itself the energy balance reflects no limitations or restrictions on conversion of energy from any form into any other form. As
a simple example, consider that heat transfer from a colder object to a hotter object
does not violate the first law of thermodynamics, but we know from experience that
such a process does not occur. Entropy is a property that allows us to incorporate
realistic limitations like this one into our mathematical models of thermodynamic
systems and processes.
We begin our examination of entropy by introducing the concept of the reversible
process, and how it relates to the notions of “maximum work” and “minimum work.”
4.2 reversible Processes
The attributes that
characterize an ideal
gas, listed in Section
2.3.3, are never literally true for real gases.
Similarly, these four
statements will never be
literally true for any real
process. But the “ideal
gas” and the “reversible
process” are often useful models.
This section defines what a reversible process is, and explains why the concept is useful. Briefly, a reversible process is identified as follows:
1. A process in which all driving forces for change are infinitesimally small.
2. A process that progresses through a series of equilibrium states.
3. A process that can be returned to its original state with no NET addition of heat
or work.
4. A process that has no friction.
Reversible and irreversible processes are illustrated in Examples 4-2 and 4-3, where
these attributes of a reversible process are explained in detail.
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C H A P T E R 4 Entropy
135
Consider a gas in a piston-cylinder device. Initially, the gas is at T = 300 K and
P = 5 bar, while the surrounding atmosphere is at T = 300 K and P = 1 bar. Because
the gas is at higher pressure than the surroundings, the gas has the potential to produce mechanical work.
Suppose the gas expands until it is in equilibrium with the surroundings, reaching a final state of T = 300 K and P = 1 bar. There are many ways this expansion
could occur. Two of the possibilities are illustrated below. Example 4-2 is an irreversible expansion and Example 4-3 is a reversible expansion. We will see that the reversible path proves to produce more work.
irreversibLe exPansiOn
Two moles of an ideal gas are confined in a piston-cylinder device, initially at P = 5 bar
and T = 300 K. The surroundings are also at T = 300 K. The atmosphere is at P = 1
bar but clamps hold the piston in place, as shown in Figure 4-2. When the clamps are
removed, the gas expands rapidly—so rapidly that heat transfer is negligible during this
step—until the pressure of the gas is P = 1 bar. Heat is then transferred to the gas from
the surroundings until the gas reaches a final state of P = 1 bar and T = 300 K. Find the
total work done by the gas on the surroundings, and the total heat transferred to the gas.
P 5 1 bar
T 5 300 K
P1 5 5 bar
N1 5 2 mol
T1 5 300 K
P 5 1 bar
T 5 300 K
P2 5 1 bar
N2 5 2 mol
T2 5 300 K
examPle 4‑2
FOOd FOr
thOught
4-2
As you read this
example, try to identify
how this process
violates any or all of
the four characteristics
of reversible processes
that were listed
previously. This will be
discussed further after
Example 4-3.
You might have experienced gases cooling
when they expand;
perhaps you’ve felt
air escaping from a
tire and noticed it’s
cool. One “first law”
explanation of this
phenomenon is that
as the gas expands, it
transfers energy to the
surroundings in the
form of work.
Figure 4‑2 Irreversible expansion of a gas as discussed in
Example 4-2.
SOLUTION: Before beginning, we note that this is computationally a fairly simple
problem, but if the system and process aren’t defined carefully, one could easily make
it more complicated than it needs to be. We are asked to find Q and W. The problem
statement describes a two-step process:
1. An adiabatic expansion from P = 5 bar to P = 1 bar, during which a temperature
drop occurs.
2. Isobaric heating of the gas to 300 K.
Your first instinct might be to calculate Q and W for each step, and add them
together to find the total Q and W for the process. But we will see that this two-step
approach is neither necessary nor efficient.
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136 Fundamentals of Chemical Engineering Thermodynamics
Step 1 Define a system
Q and W for the gas is what we’re asked to compute, so we define the gas as the system.
Step 2 Simplify the energy balance
We use the time-independent energy balance (no information on duration of process).
The gas is a closed system with no moving parts (no shaft work) and changes in kinetic
and potential energy are negligible. Thus, as in Examples 3-9 and 3-10, the energy balance
simplifies to:
M2Û2 2 M1Û1 5 WEC 1 Q
Internal energy is a
state property. Therefore, the internal
energy of the final
state is determined
only by the temperature
of the final state and
is independent of the
temperature at any
previous time. While
temperature isn’t
uniform throughout
the entire process, the
initial and final
temperatures are
identical.
(4.4)
Step 3 Envisioning the process
Examining the first step of the process only, we know Q = 0 and we know work is done
by the system on the surroundings. Thus we expect WEC to be negative, which means that
U2 is smaller than U1, which in turn means that the temperature decreases. However, the
heat capacity of the gas is not given, so there is currently no way to quantify this temperature change.
Instead, we can apply the energy balance to the entire process, looking only at the
initial and final states. This is a closed system, so the mass is constant (M2 = M1 = M) and
can be factored out:
MsÛ2 2 Û1d 5 WEC 1 Q
(4.5)
The initial and final temperatures are both 300 K. For an ideal gas, the specific internal
energy (Û) is a function of temperature only. Thus, while Û is not constant throughout the
process, it is the same at the initial and final states sÛ1 5 Û2d. Consequently the energy
balance for the entire process simplifies to:
WEC 5 2Q
FOOd FOr
thOught
4-3
Suppose we made a
plot of the internal energy of the system vs.
time. Sketch what this
plot would look like.
(4.6)
Step 4 Calculating expansion work
It was established in Section 1.4.3 that expansion/contraction work is given by:
#
WEC 5 2 P dV
P is the pressure opposing the motion, while V is the volume of the system. Here, the
gas is expanding, and the pressure opposing the motion is constant at P = 1 bar. Consequently the expansion work is
WEC 5 2s1 bardsV2 2 V1d 5 s1 bardsV1 2 V2d
A real gas wouldn’t
necessarily be well
described by the ideal
gas law at P = 5 bar.
The ideal gas model is
used for simplicity in
Examples 4-2 and 4-3
to illustrate reversible and irreversible
processes.
(4.7)
The ideal gas law is used to evaluate V2 and V1:
1 P 2 P 2
WEC 5 s1 bard
WEC 5 s1 bard
3
1
s2 mold 8.314
2
N1 RT1
N2 RT2
1
2
J
s300 Kd
mol ? K
5 bar
1
s2 mold 8.314
2
(4.8)
2
J
s300 Kd
mol ? K
1 bar
4
523991 J
This is physically reasonable in that W should be negative; the system is doing work on
the surroundings.
Step 5 Solve energy balance
Substituting the result of step 4 into the energy balance (Equation 4.6) gives Q = 3991 J.
Example 4-3 shows that an expansion with the same initial and final states as
Example 4-2 can produce a very different result for Q and W.
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C H A P T E R 4 Entropy
reversibLe expansiOn
Two moles of an ideal gas are confined in a piston-cylinder device, initially at P = 5 bar
and T = 300 K. The atmosphere is at P = 1 bar but there is a pile of sand on top of the
piston, as shown in Figure 4-3. The mass of the sand is sufficient to make the total downward pressure on the piston 5 bar. The grains of sand are removed one at a time, leading
to a gradual decrease in pressure and a gradual expansion of the gas, until the gas is at
P = 1 bar. The temperature of the gas is T = 300 K throughout.
Sand
Sand
137
examPle 4‑3
FOOd FOr
thOught
4-4
List the ways this process is different from
the one in Example
4-2, and consider how
your list relates to
the four properties of
reversible processes
given at the beginning
of this section.
P2 5 1 bar
N2 5 2 mol
T2 5 300 K
P1 5 5 bar
N1 5 2 mol
T1 5 300 K
Figure 4‑3 Reversible expansion process described in Example 4-3.
SOLUTION:
Step 1 Compare/contrast with Example 4-2
As in Example 4-2, we can define the gas as the system. This is a closed system with no
moving parts, and changes in potential and kinetic energy can be neglected, so again the
energy balance is
M2Û2 2 M1Û1 5 WEC 1 Q
As in Example 4-2, the gas is an ideal gas, so Û is only a function of temperature.
Therefore Û is the same at the start and end of the process (T1 = T2 = 300 K). Thus,
M2Û2 2 M1Û1 5 0, which in turn means
WEC 5 2Q
Step 2 Evaluating the external pressure
The work of expansion/contraction is given by:
#
WEC 5 2 P dV
“Slow” and “gradual”
are key words that
might lead you to
consider modeling
a process as reversible. However, the
definitive way to
decide whether it is
reasonable to model a
process as reversible is
comparing the process
to the criteria listed at
the beginning of this
section.
Work and heat are not
state properties, so we
cannot assume these
are the same as they
were in Example 4-2
based on the initial
and final states being
the same.
(4.9)
P is the pressure opposing the motion, which here means the external pressure pushing
down on the piston. Unlike Example 4-2, this pressure is not constant; it starts at P = 5
bar and gradually decreases to P = 1 bar.
In order to integrate Equation 4.9, we must express P as a function of V. V represents
the volume of the system, which is the gas. If P is the gas pressure, it could be related to
V through the ideal gas law. But P represents the external pressure.
The key insight comes from the description of the process as gradual. The grains of
sand are being removed one at a time. When a grain is removed, the external pressure
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138 Fundamentals of Chemical Engineering Thermodynamics
The energy balance
is Q = 2WEC. Every
time a grain of sand
is removed, a tiny
amount of work is
done (dWEC ) and an
equivalent amount of
energy is added to the
system as heat (dQ).
Q is the sum of all
these tiny heat transfers (Q = ∫ dQ).
is incrementally smaller than the gas pressure, and the gas expands incrementally, equalizing the pressure. This expansion (dV) leads to an incremental decrease in the gas temperature. Since the surroundings are now incrementally warmer than the gas, a very small
amount of heat (dQ) is added to the gas, equalizing the temperatures. Then the next grain
is removed. As grains are removed, the external and internal pressures both gradually
decrease from 5 bar to 1 bar. However, at any specific instant in time, the external and
internal pressures are essentially identical; the mass of a single grain of sand accounts for
the entire difference between them. Consequently, in modeling the process it is reasonable to equate the external pressure to the gas pressure, which means we can relate P to
V through the ideal gas law:
WEC 5 2
One of the descriptors for a reversible
process is that it
progresses “through a
series of equilibrium
states.” The discussion
in Step 2 illustrates an
example of what that
means.
# V dV
NRT
(4.10)
Step 3 Calculate expansion work
In this process, N, R, and T are all constant, so the integration is
WEC 5 2
#
V5 V2 NRT
V5 V1
V
dV 5 2NRT ln
1V 2
V2
(4.11)
1
While we can calculate V1 and V2 using the ideal gas law, it actually isn’t necessary. Since
we know P and T at both the beginning and end of the process, we can substitute for V1
and V2 for
1 2
5 2NRT ln
2
1
NRT2
WEC 5 2NRT ln
P2
NRT1
1P 2
P1
(4.12)
2
P1
1
WEC 5 2s2 mold 8.314
2
J
5 bar
s300 Kd ln
5 28029 J
mol ? K
1 bar
Step 4 Close energy balance
Substituting WEC into the energy balance from step 1, we find Q = 8029 J of heat was
added during the course of the process.
FOOd FOr
thOught
4-5
The discussion so far
suggests that a reversible process is “better”
than an irreversible
one; it produces more
work. Do you see any
advantages for the
irreversible process
(Example 4-2) compared to the reversible
one (Example 4-3)?
In both processes described in Examples 4-2 and 4-3, the initial and final states of
the process were the same. We showed mathematically that more work was actually
done in the reversible expansion. Physically, we can rationalize this result by comparing the physical effects of the processes. In both processes, the piston moved upward
and displaced the atmosphere. In both cases, the air exerted a downward pressure
of 1 bar on the piston, the gas expanded to the same final volume, and the work
required to “push the atmosphere up” was the same. But in Example 4-2, pushing
the atmosphere up was the only work done. In Example 4-3, more work was done:
in addition to pushing up the atmosphere, the piston also lifted the grains of sand.
At the start of this section we listed four attributes of reversible processes. We now discuss each in more detail, with reference to Example 4-2 and
Example 4-3.
1. a reversible process is a process in which all driving forces for change are
infinitesimally small.
Example 4-3 illustrates this aspect of a reversible process through two physical
phenomena: expansion and heat transfer.
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C H A P T E R 4 Entropy
139
The driving force for expansion is a difference in the upward and downward
pressures acting on the piston, but the difference in pressure at any given time
is very small—the weight of a single grain of sand accounts for the difference.
Therefore, in building a mathematical model of the process, it is valid to assume
Pexternal = Pinternal at any particular time.
The driving force for heat transfer is a difference in temperature between the
gas and the surroundings, but the difference in temperature is very small; it is
valid to assume that the gas and the surroundings are both at a temperature of
300 K throughout the entire process.
2. a reversible process is a process that progresses through a series of equilibrium states.
The FINAL state in both Example 4-2 and Example 4-3 is easy to recognize as an
equilibrium state; both the gas and the surroundings are at P = 1 bar and T = 300
K. But the gas in Example 4-3 can be regarded as “at equilibrium” for the entire
process.
The definition of a system at equilibrium requires that there be no driving force
for change. It might seem counter-intuitive to describe a system that undergoes a
pressure change from 5 bar to 1 bar as “at equilibrium.” But the external and internal pressures acting on the gas were never significantly different from each other.
Thus, in a reversible process, the system can be modeled as in equilibrium at any
particular moment. The system progresses very slowly from one “equilibrium state”
to another.
By contrast, an irreversible process is one in which there is at least one driving force for change that is significant in magnitude. In Example 4-2, when the
blocks holding the piston in place were withdrawn, the pressure pushing upward
on the piston was 5 bar while the pressure pushing downward was 1 bar. This was
a large driving force that led to rapid, irreversible expansion, and the only times
this process can be modeled as “at equilibrium” are the beginning––before the
blocks were removed––and the end––when the gas reached the same T and P as
the surroundings.
Examples 4-2 and 4-3
illustrate that Q and
W are path-dependent
properties, but the
final equilibrium state
(T2 = 300 K, P2 = 1
bar, U2 = U1) was the
same regardless of the
path taken to reach
that state.
3. a reversible process is a process that can be returned to its original state with
no net addition of heat or work.
The word “reversible” is descriptive in that it only takes a fractional change in driving force to cause the process to reverse direction. In Example 4-3, we imagined
causing a gas to expand by removing grains of sand from the piston one at a time,
but we could at any point have caused the gas to compress by replacing the grains of
sand one at a time.
Comparing Examples 4-2 and 4-3 demonstrates that the starting and ending conditions of the gas were identical, but the reversible pathway produced
more work. Indeed, one can demonstrate that it is always true that reversible
expansions produce maximum work. Recall that work of expansion/contraction
is given by
# P dV
WEC 5 2
V2
V1
where P is the pressure opposing the motion. So in an expansion, the larger the
external pressure, the larger the work. External pressure cannot be greater than
the system pressure. If it were, compression would occur instead, and the work
FOOd FOr
thOught
4-6
Here we have illustrated maximum work
produced by an
expansion. What kind
of example could
we use to illustrate
the minimum work
required for a
compression?
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140 Fundamentals of Chemical Engineering Thermodynamics
would be positive (work done on system) rather than negative (work done by system). Since external pressure cannot exceed internal pressure, work is maximized
when external pressure is equal to internal pressure, which is the case in a reversible expansion.
These observations generalize to processes that produce shaft work as well.
For any process that produces work, the maximum work is obtained from a
reversible pathway
For any process that requires work, the minimum work is required by a reversible
pathway
Examples 4-2 and 4-3
had different values of
Q and different values
of W, though Q + W
was equal to zero for
both examples. The
sum Q + W had to
be the same for both
because it was equal
to ΔU, which is a state
function.
FOOd FOr
thOught
4-7
Thinking back to the
Motivational Example,
consider a reversible
turbine. What would
need to be true for it
to be considered reversible? Does it seem
plausible that a reversible turbine could
actually be built?
FOOd FOr
thOught
4-8
Force and pressure
are two different
things. How can a frictional “force” equal
0.1 bar of pressure?
Friction is a force
that opposes motion
of the piston in either
direction.
Let us now further consider reversing the processes described in Examples 4-2 and
4-3. Because the internal and external pressures are essentially the same at any
given time on a reversible path, the work required to reversibly re-compress the
gas to its original pressure would be equal in magnitude and opposite in sign to the
work produced when it expanded. In Example 4-3, we computed W = −8029 J and
Q = +8029 J for an isothermal reversible expansion of the gas from P = 5 bar to
P = 1 bar. If the gas were compressed from P = 1 bar to P = 5 bar, again by an
isothermal reversible path, the work required would be W = +8029 J and the heat
released would be Q = 28029. The system would be restored to its original state,
and the NET work and heat exchanged with the surroundings would be 0. By contrast, the irreversible process in Example 4-2 did only 3991 Joules of work on the
surroundings (W = 23991 J), but at least 8029 Joules of work would be required to
return the gas to its original state. In sum:
In a reversible process, the system and surroundings can BOTH be returned to
their original state.
In an irreversible process, the system and surroundings cannot both be returned
to their original state, because returning the system to its original state would
require a net addition of heat and/or work.
4. a reversible process is frictionless.
Throughout this book, we have assumed that the piston in a piston-cylinder
apparatus is frictionless. Significant friction between the piston and the cylinder
would serve as an additional force opposing motion in either direction. Suppose,
for example, that the force of friction was equivalent to 0.1 bar of pressure acting on the piston. As Figure 4-4 illustrates, the final pressure of the system would
be 1.1 bar rather than 1 bar: at this point, the driving force for change still exists
but is no longer sufficient to overcome friction. Thus, there is no further motion
of the piston beyond this point. In a frictionless process the gas would continue
expanding until its pressure was 1 bar, thus doing more work. Figure 4-4 further
illustrates that in order to get the process to reverse direction, one would need
to change the applied force by at least 0.2 bar. Recall that in a reversible process,
an infinitesimal change in driving force is sufficient to change the direction of the
process.
This section started with four identifying attributes of a reversible process. We
have now considered the significance of each of these in the context of reversible and
irreversible processes. The concept of the reversible process is fundamentally necessary for defining, understanding, and quantifying the property of entropy, which we
introduce formally in the next section.
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C H A P T E R 4 Entropy
P 5 1 bar
141
P 5 1.2 bar
Friction
Friction
P 5 1.1 bar
P 5 1.1 bar
Internal
pressure
Internal
pressure
(a)
(b)
Figure 4‑4 A piston cylinder with frictional force equivalent to 0.1
bar of pressure. (a) The internal pressure is 0.1 bar greater than the
external pressure, but this isn’t enough of a driving force to overcome
the force of friction, so no further motion occurs. (b) In order to
compress the gas, the external pressure must be increased until it
exceeds the internal pressure by more than 0.1 bar.
4.3 defining and describing entropy
Examples 4-2 and 4-3 described a gas confined in a piston-cylinder arrangement.
Because the gas pressure (P = 5 bar) was initially significantly greater than the
external pressure, one might say the gas had the potential to do mechanical work.
Because the final temperature and pressure of the gas were in equilibrium with
the surroundings, the gas at this point had no more potential to do mechanical work,
unless there was first some kind of outside intervention (adding heat, adding work,
moving the cylinder to surroundings where the external pressure was lower, etc.).
Example 4-3 demonstrated that the gas was capable of doing 8029 Joules of
work before it reached an equilibrium state with the surroundings. In Example 4-2,
the initial and final states were the same as in Example 4-3, so the potential to do
work was the same; however, in reality the work produced (3991 J) was less than half
of the potential work. One might therefore say there were over 4000 Joules of “lost
work.” Entropy is a property that allows one to quantify “lost work.”
The second law of thermodynamics can be stated as follows. The entropy of the
universe cannot decrease. The entropy of the universe is unchanged by any reversible
process and increased by any irreversible process.
Like the first law of thermodynamics, this “law” is reflected consistently in observation and experience. The second law won’t seem particularly helpful until you
develop more insight into what entropy actually is. Therefore, we present several
essential facts about entropy here.
In Examples 4-2 and
4-3, the entropy change
of the gas was the
same; the gas experienced the same decrease in potential to do
work. In the reversible
case the potential work
was 100% converted;
in the irreversible case
less than 50% of the
potential work was
harnessed.
In 1865, Rudolf
Clausius famously
summarized the first
two laws of thermodynamics as follows:
“The energy of the
world is constant. The
entropy of the world
tends toward a maximum.” (Olson, 1998)
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142 Fundamentals of Chemical Engineering Thermodynamics
At some point, you
may have heard a
definition like
“entropy is a measure
of disorder.” This
will be addressed in
Section 4.3.5.
Entropy is a state property. The change in entropy experienced by the gas in
Example 4-2 and Example 4-3 must therefore be the same, since the initial and
final states are the same.
The entropy of the universe is unchanged by a reversible process. Entropy can
be viewed as a property that quantifies “lost work.” There is no lost work in a
reversible process: the maximum work possible for a particular initial and final
state is obtained if the process produces work, and the minimum possible work
is used by a process that requires work.
The entropy of the universe never decreases. Qualitatively this observation can
be rationalized in that “lost work” can only be zero or positive; actual work
can’t exceed potential work. A more mathematical demonstration of the fact
that entropy never decreases is given in Example 4-5.
The entropy of the universe increases during any irreversible process, as illustrated in Examples 4-5 and 4-9. The larger the increase in entropy, the more
potential work was lost.
While the entropy
change of the gas was
identical in Examples
4-2 and 4-3, Example
4-4 will show that
the entropy change
of the universe was 0
in the reversible case
and positive in the
irreversible case. This
reflects the fact that,
in the reversible case,
there was no “lost
work.”
A “homogeneous”
system is uniform in
temperature, pressure,
and composition. A
system that contains
distinct components
or regions, such as an
ice–water mixture, is
not homogeneous.
FOOd FOr
thOught
4-9
Entropy was introduced as a property
that allows us to quantify “lost work,” yet
work doesn’t appear
in Equation 4.13 or
4.14. Is there a contradiction here?
Notice that several of the above statements refer to the entropy “of the universe.”
When we are doing a problem, “the universe” essentially means “the system AND
the surroundings.” When thinking about entropy it is important to distinguish
among “the system,” “the surroundings” and “the universe.” The change in entropy
for a system can be negative, zero, or positive, but if it’s negative, then according to
the second law of thermodynamics there must be a corresponding increase in the
entropy of the surroundings. Consider, for example, a cup of water left outside on
a cold winter night. In the morning the water will be ice, and ice has lower specific
entropy than liquid water at the same pressure. Thus the entropy of the water in the
cup has decreased, and the next section introduces the mathematics that would allow
you to quantify this decrease. But the entropy of the universe has not decreased—
it has increased through the heat transfer from the water to the surrounding air.
Example 4-5 and Example 4-9 illustrate how heat transfer processes lead to an
increase in the entropy of the universe.
4.3.1 Mathematical definition of entropy
Quantitatively, the change in entropy for a closed, homogeneous system is defined as
dS 5
dQrev
(4.13)
T
In integrated form, it is
Sfinal 2 Sinitial 5
#
final
initial
dQ rev
T
(4.14)
where
S represents the total entropy of the system.
dS represents a differential change in the entropy of the system.
Qrev is the heat energy added to the system on a reversible path.
T is the absolute temperature of the system at the boundary where the heat
transfer occurs.
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C H A P T E R 4 Entropy
143
Because Equation 4.14 involves heat, which is a path-dependent property, it is not
obvious from the definition that entropy is indeed a state property. The crucial point
is that dQrev in Equation 4.14 represents the reversible heat; the heat transferred on
a reversible path. It can be proved that while Q is not a state function, ∫dQrev∙T is a
state function. A full proof will not be shown here, but it can be done by demonstrating that ∫dQrev∙T = 0 for any cyclic process (Gutman, 1988) (Vemulapalli, 1986). If
S always returns to its original value at the end of any cyclic process, then its value
must be dependent only upon the original (and final) state of the system, and not
upon the path.
Equation 4.14 can only be applied directly to reversible paths. Thus, in order
to use this expression to calculate the change in entropy resulting from an irreversible process, one must take advantage of the fact that entropy is a state property:
the change in entropy is a function only of the initial and final states, and is not
dependent upon the path. Consequently, one can envision a hypothetical reversible
path between the initial and final states of interest, and apply Equation 4.14 to this
hypothetical path.
Common conceptual errors that can occur when one uses Equation 4.14
include:
One cannot integrate the equation directly to ΔS = Q∙T unless the system temperature is constant. If temperature is not constant, or if one does not know
whether it is constant, one must find a mathematical relationship between Q and
T before one can integrate. This relationship typically comes from the energy
balance, as illustrated below in Example 4-4.
For an irreversible process, one can fall into the trap of using the actual Q
from the real path in Equation 4.14, rather than constructing a hypothetical
reversible process. Example 4-4 demonstrates the potential consequences of
this error.
For a reversible process, one can fall into the trap of not applying Equation 4.14
at all, instead saying to oneself “This is a reversible process so I know the change
in entropy is zero.” The change in entropy of the universe is indeed zero for a
reversible process, as illustrated in Example 4-4. But the change in entropy of a
system can be positive, zero, or negative.
change in entrOPy FOr an exPansiOn
examPle 4‑4
Revisit the two processes described in Examples 4-2 and 4-3. Applying Equation 4.14,
calculate the changes in entropy for the gas, the surroundings, and the universe for both
processes.
SOLUTION: Table 4-1 summarizes the processes described in Examples 4-2 and 4-3,
which involved gases confined in a piston-cylinder device.
Step 1 Apply definition of entropy to the gas in the reversible expansion
The mathematical definition of entropy is:
S2 2 S1 5
#
state 2 dQ rev
state 1
T
(4.15)
The process in Example 4-3 is a reversible process, so the value of Q computed in
Example 4-3 can be used directly in Equation 4.15. Further, T was constant in this process,
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144 Fundamentals of Chemical Engineering Thermodynamics
tabLe 4-1 Comparison of reversible and irreversible expansions of ideal gas.
reversible
(example 4-3)
irreversible
(example 4-2)
Initial gas pressure (P1)
5 bar
5 bar
Final gas pressure (P2)
1 bar
1 bar
Initial gas temperature (T1)
300 K
300 K
Final gas temperature (T2)
300 K
300 K
Work (W )
−8029 J
−3991 J
Heat (Q)
+8029 J
+3991 J
Change in entropy, system (DSsys)
26.8 J/K
26.8 J/K
Change in entropy, surroundings (DSsurr )
226.8 J/K
−13.3 J/K
0
13.5 J/K
Change in entropy, universe (DSuniv )
so Equation 4.15 integrates to:
S2 2 S1 5
PitFaLL
PreventiOn
For the irreversible
process,
Q 3991 J
J
5
5 13.3
T
300 K
K
However, this is not
the correct change
in entropy for the
gas, because Q is
not calculated for a
reversible path.
Qrev
T
5
J
8029 J
5 26.8
300 K
K
(4.16)
Step 2 Apply definition of entropy to gas in irreversible expansion
What of the irreversible process illustrated in Example 4-2? Since this is not a reversible process, we cannot introduce the Q value calculated in Example 4-2 directly into
Equation 4.14. In order to apply Equation 4.14, we can construct a hypothetical
reversible path that converts the system from the initial state to the final state, and then
evaluate Q on this hypothetical path. Because the initial and final states of the gas are the
same in Examples 4-2 and 4-3, our reversible process in Example 4-3 IS a path we could
apply to Example 4-2. Thus, the change in entropy of the gas is the same in both examples.
We could have arrived at the same conclusion by noting that entropy is a state function.
Step 3 Apply definition of entropy to surroundings in reversible expansion
First, consider how the processes affected the surroundings. Some work was transferred
from the gas to the surroundings, but work does not appear in Equation 4.14. Some
heat was transferred from the surroundings to the gas, though this heat transfer was not
significant enough to have a measurable impact on the temperature of the surroundings.
Therefore, Q for the surroundings is identical in magnitude, and opposite in sign, to Q
for the gas. The temperature of the surroundings is constant at 300 K. For the reversible
process in Example 4-3, Equation 4.14 applies directly:
S2 2 S1 5
Qrev
T
5
J
28029 J
5 226.8
300 K
K
(4.17)
Step 4 Apply definition of entropy to surroundings in irreversible expansion
On the irreversible path, Q = 23991 J, but the expansion and heat transfer were not
reversible. The temperature of the surroundings was 300 K throughout the process. The
temperature of the gas started and ended the process at 300 K, but we know it was lower
in between (though we never computed any exact values for it). Similarly, the expansion
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C H A P T E R 4 Entropy
was irreversible because the surroundings were uniformly at P = 1 bar while the gas
pressure varied between 1 and 5 bar. However, the effect on the surroundings would have
been the same if the heat transfer was reversible. We pretend the system was at 1 bar and
300 K throughout the process, as shown in Figure 4-5, and this gives us a “hypothetical
reversible path” of the process- from the perspective of the surroundings.
S2 2 S1 5
Qrev
T
5
23991 J
J
5 213.3
300 K
K
P 5 1 bar, T 5 300 K
(4.18)
If you calculate a
negative change in
entropy for the universe, then either
you’ve made an error,
or the process you are
modeling is unrealistic.
P 5 1 bar, T 5 300 K
WEC out
P 5 1 bar
T 5 300 K
P 5 5 bar
T 5 300 K
Qin
(a) Actual path
P 5 1 bar, T 5 300 K
P 5 1 bar, T 5 300 K
P 5 1 bar, T 5 300 K
dWEC out
dWEC out
P 5 1 bar
T 5 300 K
P 5 1 bar
T 5 300 K
P 5 1 bar
T 5 300 K
dQin
145
dQin
(b) Hypothetical reversible path
Figure 4‑5 The (a) actual process, and (b) a hypothetical reversible
process that has the same effect on the surroundings. The amount of
work done on the surroundings and the total heat transferred from the
surroundings is the same whether the process is sudden (as in a) or
gradual (as in b).
Step 5 Calculate change in entropy of the universe
The change in entropy of the universe is simply the sum of the changes in entropy for
the system and surroundings: DSuniverse = DSsystem + DSsurroundings. Thus, DSuniverse is 0 for the
reversible process and +13.5 J/K for the irreversible process.
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146 Fundamentals of Chemical Engineering Thermodynamics
In perusing Example 4-4, you might be confused about one thing: On the irreversible path, Qreversible was the same as the actual Q when we modeled the surroundings,
but the reversible and actual values of Q were different when we modeled the gas.
This apparent contradiction arises because the gas was substantially affected by the
process, while the surroundings were not.
The gas experienced a dramatic pressure change, and this change had to be modeled reversibly in order to apply Equation 4.14.
The surroundings remained at atmospheric pressure and 300 K throughout the
process. Since the surroundings were substantially unaffected by the process, the
hypothetical reversible path for the surroundings is not meaningfully different
from the actual process.
This example illustrates a common situation: the surroundings are often modeled
as so much bigger than the system that the process has no measureable effect on
the surroundings. As a simple analogy, consider a person inflating a bicycle tire in a
closed garage:
FOOd FOr
thOught
4-10
Is the water in a car’s
radiator a “heat sink,”
and can you model it
as a “heat reservoir”?
Theoretically, as the tire gets bigger, it transfers energy to the air in the garage in
the form of work.
Theoretically, the air pressure in the garage increases, because now that the tire
takes up more space, the volume of air in the garage is smaller.
Practically speaking, neither of the above effects is measurable or significant.
The bike tire is significantly different at the end of the process than it was at the
start, but the garage hasn’t changed in a significant way.
A heat reservoir is a constant temperature source, or sink, for energy. The reservoir
is assumed to be so large that its temperature will not change measurably, no matter
how much heat is transferred to or from the reservoir. As illustrated in Example 4-4,
the entropy change for a heat reservoir is given by
DSreservoir 5
Qreservoir
Treservoir
(4.19)
4.3.2 qualitative Perspectives on entropy
In this chapter, entropy was introduced as a metric for “lost work.” Increases in
entropy occur (a) when a task that requires work is accomplished in a manner that
exceeds the minimum work, or (b) when a process produces less work than it had
the potential to produce.
You may have heard other nutshell descriptions and definitions of entropy, such
as “entropy is a measure of disorder.” The “lost work” descriptor is emphasized in
this chapter for several reasons.
In Example 4-1,
processes A, B, and
D involved decreasing the entropy of the
universe, meaning
the “lost work” was
negative; the work
exceeded what was
physically possible.
The concept of “work” has recognizable practical importance to engineers, and
by identifying and quantifying “lost work” we can compute efficiencies.
The notion that entropy is a metric for quantifying “lost work” is logical from a
historical perspective; entropy was first proposed as a property in the development of quantitative models for heat engines.
The idea of “lost work” helps emphasize the complementary nature of energy
and entropy. Recall the motivational example in Section 4-1, which examined
a turbine that took in superheated steam and expelled liquid water, producing
enormous amounts of work without violating the energy balance. The entropy
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C H A P T E R 4 Entropy
147
balance allows us to recognize such a process as impossible, benchmark how
much potential work exists, and quantify how much of this potential is realized.
However, the following sections examine the concepts of entropy and reversibility
from additional perspectives in order to provide deeper insights into entropy and
also to help the reader reconcile any apparent contradictions between the discussion
in this chapter and anything he/she has heard before.
4.3.3 relating entropy to spontaneity and directionality
It has been observed that the entropy of the universe is constantly increasing. Thus,
the concept of entropy is useful in that it permits us to predict whether proposed
processes will or will not occur spontaneously. We illustrate the concepts of spontaneity and directionality with a simple example.
heat transFer bet Ween hOt and cOLd Water
examPle 4‑5
“Vessel A” contains 100 g of water initially at 80°C and “Vessel B” contains 50 g of water
initially at 20°C. They are brought into contact as shown in Figure 4-6, and allowed to
reach equilibrium. Find the final temperature of the water and the change in entropy of
the universe. Assume the heat capacity and specific volume of water are constant with
respect to temperature (use ĈP ~ ĈV ~ 4.2 J/g · K), and that no heat exchange with the
surroundings occurs.
A
B
100 g water
50 g water
T 5 808C
T 5 208C
A
B
100 g water
50 g water
T5?
T5?
Figure 4‑6 Two containers of water are
brought into contact and heat transfer occurs as
examined in Example 4-5.
SOLUTION:
Step 1 Define the system
A possible first thought is to define BOTH containers of water as the system. For this system, there is no heat entering or leaving (Q = 0), so on a first glance, Equation 4.14 might
appear to imply that there was no change in entropy. However, a closer look reveals two
problems with this analysis:
Neither ĈP nor V̂
for liquid water is
precisely constant
over the temperature
range from 208C to
808C, but here it is
convenient to use
these simplifying
approximations for
illustrative purposes.
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148 Fundamentals of Chemical Engineering Thermodynamics
1. Equation 4.14 applies to homogeneous closed systems. A system can’t be considered
homogeneous if there are distinct regions with two different temperatures inside the
system.
2. Equation 4.14 applies only to reversible paths. If we define both containers as the
system, the process is not reversible—there is an irreversible heat transfer process
occurring inside the boundaries of the system.
We will find it much more useful to define ONE container of water as the system; now we
do have a homogeneous closed system. Further, the other container is outside the system
and there is heat (Q) entering or leaving the system. Thus, we will be able to apply Equation 4.14 to each container.
Consequently, we define two distinct systems, A and B, each consisting of the water
in one vessel.
FOOd FOr
thOught
4-11
Give both a physical
and a mathematical
explanation for how
Equation 4.20 can be
derived from
Equation 3.35.
dÛ 5 ĈV dT because
this is being modeled
as a constant-volume
process.
Step 2 Apply and simplify energy balance
The energy balance equation and mathematical analysis for each system will be essentially identical, with only the initial temperature being different.
The containers of water are closed, stationary systems, so the energy balance for one
container of water, in differential form, is
dsMUˆ d 5 dQ 1 dWEC 1 dWS
(4.20)
Since the volume of water is assumed constant, neither system will experience expansion
or contraction (WEC = 0). There are also no moving parts (WS = 0), so the energy balance
further simplifies to
dsMÛ d 5 dQ
(4.21)
Mass is constant, and dÛ can be related to the temperature using the known, constant
heat capacity:
MĈV d T 5 dQ
(4.22)
This equation can be applied individually to either vessel of water, A or B:
MAĈV dTA 5 dQA
and
MBĈV dTB 5 dQB
(4.23)
Step 3 Integrate energy balance
To find the final temperature, we integrate Equation 4.23 from the start of the process to its end:
#
TA, final
TA, initial
MAĈV dTA 5 QA
(4.24)
Because the mass of the water in vessel A (MA) is constant and we have assumed for this
example that the heat capacity of water is also constant, this becomes
MAĈV sTA,final 2 TA,initial d 5 QA
(4.25)
The analogous treatment of vessel B gives
pitFaLL
preventiOn
Be careful with signs.
QA and QB represent
exactly the same heat
transfer, so they are
equal in magnitude,
but they are opposite
in sign.
MBĈV sTB,final 2 TB,initiald 5 QB
(4.26)
Step 4 Relate the systems to each other to find final temperature
The problem statement says that no heat transfer occurs with the surroundings. Thus, any
heat leaving vessel A must go to vessel B; there is nowhere else for it to go, and there is
no other source of heat added to vessel B. Consequently:
QA 5 2QB
(4.27)
MAĈV sTA,final 2 TA,init d 5 2MBĈV sTB,final 2 TB,init d
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C H A P T E R 4 Entropy
149
At equilibrium there is no driving force for more heat transfer; the temperatures of the
vessels are the same (TA, final = TB, final = Tfinal). Applying this fact to Equation 4.27 gives
MAĈV sTfinal 2 TA,initd 5 2MBĈV sTfinal 2 TB,initd
(4.28)
1 g ?J K2 sT 2 808Cd 5 s50 gd14.2 g ?J K2sT 2 208Cd
s100 gd 4.2
final
final
The final temperature is Tfinal = 608C.
Step 5 Apply definition of entropy
The definition of entropy is
dS 5
dQrev
T
Since this heat transfer is irreversible (initially, the temperature gradient between the
two containers is 60 degrees), we must construct a hypothetical reversible process in
order to apply Equation 4.14.
Figure 4-7 focuses on Vessel B and illustrates the actual irreversible process and a hypothetical reversible process. In the real process, the source of the heat is initially at 808C, and
this temperature decreases until equilibrium is reached at 608C. Because the heat source is
outside the boundaries of the system, we can construct a hypothetical process in which the
same amount of heat comes from a different source. In the hypothetical process, the temperature of the heat source is initially 20.18C; this means there exists a very small driving force
for heat transfer to the system, which is initially at 208C. In the hypothetical process, the
temperature of the heat source gradually increases along with the system temperature, so
that the heat source temperature is always fractionally larger than the system temperature.
Notice the energy balance in Equation 4.22 is not dependent upon the temperature of the
heat source; it’s the same for the reversible process as the irreversible one. The hypothetical
process is not very realistic, but it has the same effect on container B as the real process has.
The actual, irreversible process
A
B
A hypothetical reversible process
A
B
Q
Q
T 5 808C
T 5 208C
T 5 20.18C
T 5 20.08C
A
B
A
B
Q
Q
T 5 708C
T 5 408C
T 5 40.18C
T 5 40.08C
A
B
A
B
Q
T 5 60.18C
T 5 58.88C
The physical effect
of heat transfer (e.g.,
temperature change,
phase change, etc.) is
the same, whether the
heat is transferred
reversibly or
irreversibly.
There’s nothing
special about the
0.1 degree temperature difference
shown in Figure 4-7.
We could’ve said
the heat source was
always 0.01 degrees
warmer than the
system, or 0.001. The
crucial point is that the
temperature difference
must be small enough
that it’s reasonable to
consider it negligible;
otherwise we can’t
reasonably model the
process as reversible.
Q
T 5 58.98C
T 5 58.88C
Figure 4‑7 An illustration of the actual process described in
Example 4-5, and of a hypothetical reversible process that has the
same effect on the water in container B.
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150 Fundamentals of Chemical Engineering Thermodynamics
The rate of heat transfer is much faster in
an irreversible process
than a reversible one,
but rate isn’t important in this example.
Introducing Equation 4.22, for Q, into the definition of entropy
dS 5
MĈV dT
(4.29)
T
This equation, like the energy balance in Equation 4.22, can be applied individually to
either system.
Step 6 Solve for changes in entropy
Since mass and heat capacity are both constant, Equation 4.29 integrates to
S2 2 S1 5 MĈV ln
1T 2
T2
(4.30)
1
Vessel B has an initial temperature of 208C and a final temperature of 608C. Thus,
K
J
5 26.9
1 g ?J K2 ln 16020 11 273.15
273.15 K 2
K
S2,B 2 S1, B 5 s50 gd 4.2
Temperature must be
expressed on an absolute scale when applying the definition of
entropy, or any other
equation derived
from it.
(4.31)
For Vessel A, the initial temperature at 80°C gives
K
J
5 224.5
1 g ?J K2 ln 16080 11 273.15
2
273.15 K
K
S2, A 2 S1, A 5 s100 gd 4.2
(4.32)
Since the process has no interaction with anything outside these two containers,
the change in entropy of the universe is the sum of these two entropy changes:
ΔSuniv = 26.9 2 24.5 = 2.4 J/K.
Example 4-5 illustrates how irreversible heat transfer always leads to an increase in
the entropy of the universe. Heat always flows from a high-temperature location to a
lower-temperature location. Thus, Q is negative for the system with the larger initial
T, and Q is positive for the system with the smaller initial T. So when the values of
Q/T are summed, the result is positive.
When an object at 808C is in contact with an object at 208C, no action is required
to make heat transfer begin; it begins predictably and spontaneously. Thus, the process illustrated in Figure 4-6 can be termed a spontaneous process. One can also imagine the reverse process shown in Figure 4-8, in which all of the water is at 608C initially,
and one container increases in temperature to 808C while the other drops to 208C.
Such a process is consistent with the first law of thermodynamics (energy is conserved
in Figure 4-8 whether the process runs “forward” or “backward”) but is not consistent
with the second law (the process in Figure 4-8 would decrease the entropy of the universe). Thus, the process shown in Figure 4-8 is not a spontaneous process.
We don’t need the concept of entropy to know that heat transfers spontaneously
from a hot object to a cold object. We know this from experience. Similarly, we can
prove that the mechanical process illustrated in Figure 4-9, in which a gas increases
pressure to 5 bar even though there is no applied force acting on it besides the atmosphere, would lead to an increase in the entropy of the universe (Problem 4-15 asks
you to do this proof). Perhaps it seems like a waste of time to do this mathematical
proof; we know intuitively that such a process isn’t spontaneous. However, many
physical situations are more subtle or complex, and intuition and experience aren’t
enough. Recall that in Example 4-1, three of the four turbine configurations are
shown to be physically impossible using the concept of entropy. These turbines did
not violate the laws of nature in any way that most people would necessarily call
“intuitively obvious.” As another example, consider the gas phase chemical reaction:
C2H6 ↔ C2H4 1 H2
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C H A P T E R 4 Entropy
A
B
100 g water
T 5 808C
50 g water
T 5 208C
P 5 1 bar,
T 5 300 K
A
B
100 g water
T 5 608C
50 g water
T 5 608C
151
P 5 1 bar,
T 5 300 K
P 5 1.0 bar
T 5 300 K
P 5 5.0 bar
T 5 300 K
Figure 4‑8 The reverse of the heat transfer
process illustrated in Example 4-5. This process
is physically unrealistic, though energy is
conserved.
Figure 4‑9 A mechanical process that would not occur
spontaneously.
If 1.0 mole of ethane, 0.5 mole of ethylene, and 0.5 mole of hydrogen were placed
in a container, would the reaction spontaneously run “forward” or “backward”?
The summary answer is “It depends upon the temperature and pressure,” but we
can’t answer the question quantitatively without the concepts of both enthalpy and
entropy. Most likely, the reader has solved problems of this kind in chemistry courses,
by calculating equilibrium constants (K) using the expression:
ln K 5
2DG 0
RT
(4.33)
The compound C2H4
is called both “ethene”
and “ethylene.”
Ethene is the name
by IUPAC convention, but ethylene is
a name that predates
the IUPAC system of
nomenclature and is
still in common use.
in which DG 0 is the standard change in molar Gibbs free energy from the reaction.
This very chemical reaction is considered in the motivational example for Chapter
14, in which we examine chemical reaction equilibrium and learn the theoretical
basis for Equation 4.33. For now we simply note that Gibbs energy (G) is a fundamental property for modeling equilibrium, and is defined as follows:
G 5 H 2 TS
(4.34)
Equation 4.34 shows that entropy is embedded into G. Gibbs energy in turn is the
fundamental property that determines which direction (and how far) the reaction
will run spontaneously.
Phase changes are another crucial physical phenomenon that can be addressed
using Gibbs energy. Chapters 8 through 13 discuss how one can predict and model
the temperatures and pressures at which phase transitions occur spontaneously, and
the role entropy and Gibbs energy play in building these models.
Why do we refer to the process in Figure 4-6 as “spontaneous,” and those in
Figures 4-8 and 4-9 as “not spontaneous”? Why don’t we simply call them “possible” and “impossible”? Because one of the things engineers do is design systems
that allow non-spontaneous processes to happen! For example, heat would not
spontaneously flow from a location at ~388F to a location at ~758F, but a standard
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152 Fundamentals of Chemical Engineering Thermodynamics
kitchen refrigerator is a machine that is designed to make this exact non-spontaneous process occur. Section 5-3 describes how refrigeration cycles actually work.
For now we note that part of the refrigeration process involves the addition of
work. The addition of work is an essential feature of carrying out processes that
wouldn’t occur spontaneously. A ball will roll downhill spontaneously, but can
only be made to roll back up the hill if work is applied. Similarly, heat flows from
high temperature to low temperature spontaneously, but heat can be made to flow
“uphill” to higher temperatures through a well-designed process that involves the
addition of work.
4.3.4 spontaneity and reversibility
Like the ideal gas, the
reversible process is
a useful theoretical
model, which in some
circumstances closely
approximates reality.
Q cannot equal
AΔT; the units are different. A constant of
proportionality called
the heat transfer coefficient is needed to
make the equality.
A approaches infinity
as ΔT approaches 0.
Minimizing the cost of
a piece of equipment
and minimizing the
costs (such as energy)
associated with operating that equipment
are often competing,
contradictory goals.
Typically, chemical
engineering curricula
include design courses
that include examination of how to optimize this trade-off.
Section 4.3.3 notes that in any spontaneous process, the entropy of the universe increases. A reversible process is a limiting case in which the driving force for the
process is infinitesimally small. It was illustrated through Examples 4-2 and 4-3 that
reversible processes provide the maximum work when work is produced. There is no
“lost” work in a reversible process. It would be easy to infer from this discussion that
reversible processes are the “best” processes. However, we must also be conscious of
the rate at which processes occur. We consider the physical process of heat transfer
as an example.
The rate of heat transfer by convection or conduction is proportional to the area (A)
available for heat transfer and also proportional to the temperature gradient (ΔT )
driving the heat transfer:
·
Q ~ A DT
(4.35)
Typically, undergraduate curricula in chemical engineering contain an entire course
on heat transfer, which covers other factors influencing the rate of heat transfer
and considerations in equipment design. However, Equation 4.35 is sufficient to
illustrate the problem with reversible processes: in a reversible process, ΔT by definition approaches zero, so the heat transfer area (A) required to obtain any desired
finite rate of heat transfer (Q) must approach infinity. A reversible heat transfer is
optimal from the perspective of efficient use of energy, but equipment cost is minimized with a low heat transfer area (A), which requires a highly irreversible, highly
spontaneous process. Such trade-offs are critical in engineering design and engineering practice.
In sum, while the lost work that accompanies an irreversible, spontaneous
process might at first glance seem wasteful compared to a reversible process, one
might be better served by regarding some lost work as “the cost of doing business.”
4.3.5 relating entropy to disorder
You have likely heard that entropy is “a measure of disorder.” The description of entropy as a metric for “lost work” is emphasized in this book largely because “work”
is a physical quantity that has a formal mathematical definition. Thus, “maximum
work” and “lost work” can be quantified, while a “measure of disorder” is an ambiguous notion. However, the description of entropy as a “measure of disorder” is
justifiable, at least in a qualitative sense. Consider the following general observations
about entropy:
Specific entropy is higher in vapors than in liquids.
Specific entropy is higher in liquids than in solids.
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C H A P T E R 4 Entropy
Entropy of a multi-component mixture is higher than the sum of the entropies
of the individual components of the mixture.
Entropy tends to increase with increasing temperature.
Entropy tends to increase with decreasing pressure.
Even without a formal definition for “order,” it probably seems obvious that solids
are more “ordered” than liquids, liquids are more ordered than gases or vapors, and
pure substances are more ordered than mixtures. Furthermore, increasing temperature and decreasing pressure both typically lead to increases in volume; most people
would probably agree that something becomes less “ordered” when it is spread out
through a larger volume. So, you can probably see why “entropy is a measure of
disorder” became a fashionable description: it is a concise summary of several broad
observations regarding entropy.
When looking at more complex situations, the qualitative notions of “order”
versus “disorder” become difficult to apply in any meaningful way. Consider, for
example, the two systems shown in Figure 4-10: (a) superheated steam at 10 bar and
2008C, and (b) a system comprised of both saturated liquid and satured vapor at
1 bar. Offhand, would you say system (b) is more “ordered,” because it is partially
liquid and liquids are more ordered than vapors? Or would you say the superheated
system is more “ordered” because it’s homogeneous while the liquid–vapor system
is heterogeneous? Also, in measuring “order,” is a phase change more or less important than a change in pressure or temperature?
In fact, the two systems in Figure 4-10 have equal specific entropy. If the superheated steam in Figure 4-10a is fed into an adiabatic reversible turbine and the exit
pressure is 1 bar, the effluent from the turbine would be the liquid–vapor mixture
described in Figure 4-10b. In this scenario, there is no lost work. The reversible turbine produces the maximum possible work attainable for this particular change in
state; thus there is no change in entropy. All of these facts can be demonstrated by
applying the mathematical definition of entropy in Equation 4.14.
Ŝ v 5 7.3594
P 5 10 bar, T 5 2008C
Ŝ 5 6.694
kJ
kg ∙ K
Don’t overextend
generalizations such
as “specific entropy
is higher in liquids
than in solids.” For
example, while liquid
water has higher specific entropy than ice,
you can’t assume it has
higher specific entropy
than ALL solids.
kJ
kg ∙ K
P 5 1 bar, T 5 99.638C
Ŝ system 5 6.694
kJ
kg ∙ K
Ŝ L 5 1.3026 kJ
kg ∙ K
(a) Pure superheated steam
153
(b) 89% steam, 11% liquid water
Figure 4‑10 (a) Superheated steam and (b) a liquid–vapor mixture.
Thus, entropy is well described as a property that can be used to quantify potential work and lost work. There exist other descriptors like “entropy is a measure
of disorder” that you might find helpful in understanding or predicting changes in
entropy, at least qualitatively. But the only definition of entropy that you actually
need is the mathematical one, which is explored further in Section 4-4.
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154 Fundamentals of Chemical Engineering Thermodynamics
4.3.6 the Microscopic definition of entropy
The mathematical definition of entropy in Section 4.3.1 relies upon macroscopic
properties—specifically temperature and reversible heat. This section overviews the
fact that entropy can also be defined, and understood, at the microscopic level—the
level of individual molecules. For simplicity, here we model molecules as spheres.
The dark outer square in Figure 4-11 represents a vessel containing three “gas molecules,” labeled 1, 2, and 3. There are any number of arrangements for these molecules, just one of which is shown in Figure 4-11. How might we describe the positions
of the molecules?
2
3
1
Figure 4‑11 Three gas “molecules”
in a “vessel” that has a fixed volume.
There is nothing
“special” about our
decision to divide
the space into four
regions—we can
divide the space into
as few or as many
regions as we wish, as
necessary for whatever model we are
trying to build.
Let’s divide the “vessel” up into four regions with equal volumes I, II, III, and IV.
There are 4 3 4 3 4 = 64 different ways to distribute the molecules into the regions,
since there are four possible locations for molecule 1, and for each of these four possibilities, there are four possible locations for molecule 2, etc. We call each of these
64 distributions a “microstate.”
Three “microstates” are shown in Figure 4-12. Notice that in all three of these
“microstates,” there are exactly two molecules in region I, and one in region II. Consider this question: “Are these three really different from each other?” They look
different to us, because we’ve put numbers on the molecules, making them distinguishable. But real molecules within a pure compound are not distinguishable from
each other, so the three “microstates” in Figure 4-12 are functionally identical. “Two
in region I, one in region II” is an example of a macrostate—a specific, complete description of one possible state for the system. In this simple system of 3 molecules
and 4 regions, there are 20 different macrostates, summarized in Table 4-2. However,
these macrostates are not all equally likely to occur. There is, for example, only one
2
3
2
1
3
I II
III IV
1
2
1
I II
III IV
3
I II
III IV
Figure 4‑12 A “vessel” containing three “molecules.” The vessel is divided up into four
distinct regions, I, II, III, and IV.
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C H A P T E R 4 Entropy
155
Table 4‑2 List of the 20 macrostates for the three-molecule, four-region system
illustrated in Figure 4-12. The # of microstates sum to 64.
region i
region ii
region iii
region iv
# Microstates
3
0
0
0
1
0
3
0
0
1
0
0
3
0
1
0
0
0
3
1
2
1
0
0
3
2
0
1
0
3
2
0
0
1
3
1
2
0
0
3
0
2
1
0
3
0
2
0
1
3
1
0
2
0
3
0
1
2
0
3
0
0
2
1
3
1
0
0
2
3
0
1
0
2
3
0
0
1
2
3
1
1
1
0
6
1
1
0
1
6
1
0
1
1
6
0
1
1
1
6
microstate that has three molecules in region III; there is only one way to put all
of the molecules in the same region. However, we saw in Figure 4-12 that there are
3 different ways to put “Two in region I and one in region II”; this macrostate has
3 different microstates. While real molecules within a pure compound are indistinguishable from each other, the thought exercise of making them distinguishable
allowed us to count the number of microstates in each macrostate, and so demonstrate that some macrostates are more likely to occur by random chance than
others.
Table 4-2 shows that there are four different macrostates in which the molecules
are all in different regions. Each of these has six corresponding microstates. Therefore, if the “molecules” are being positioned randomly, these macrostates will occur
most frequently. Each of these macrostates will occur 6∙64 = 9.4% of the time,
while each macrostate in which all three molecules are in a single region will occur
1/64 5 1.6% of the time.
This discussion
doesn’t account for
the possibility that
the molecules are attracted to each other,
or the possibility that
a region could get so
full there’s no room
for more molecules.
Section 7-4 discusses
these issues.
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156 Fundamentals of Chemical Engineering Thermodynamics
Consider now adding a fourth molecule. There are now 4 3 4 3 4 3 4 = 256
different microstates. There is still only one in which all of the molecules are in
region I, only one in which all of the molecules are in region II, etc., but there are 24 different ways to put exactly one molecule in each of the four regions. What if we had
8 molecules, or 12? As the number of molecules increases, the probability of all the
molecules occupying a single region becomes infinitely small, as shown in Table 4-3.
Table 4‑3 The probability of all gas molecules occupying the same quarter of the available space by random chance.
We learned in
Section 4.3.3 that the
entropy of a system
can be decreased if we
add work––here that
could mean moving
the walls so that the
vessel is only one
fourth its original
volume.
Ludwig Boltzmann
(1844-1906) was
a physicist who
held positions at
several different
universities in Austria.
Equation 4.36 is
engraved on his
tombstone.
Fraction of microstates
in which all molecules
are in the same region
as each other
# of
Molecules
# of Microstates
# of Microstates in
which all molecules
are in the same
region as each other
4
256
4
0.015625
8
65,536
4
0.000061
12
16, 777, 216
4
2.34 3 10−7
20
1.0995 3 1012
4
3.64 3 10−12
100
1.6069 3 1060
4
2.49 3 10−60
Here, our discussion of microstates and macrostates connects to the idea of “spontaneity” discussed in Section 4.3.3. You can perhaps imagine all of the air molecules
in a room, by coincidence, moving into one quarter of the room at the same time,
leaving the other three-quarters of the room evacuated. Such an event has a small
but not insignificant frequency of 1.6% when there are only three molecules, but no
plausible frequency when there are <1025 molecules. The only “macrostates” that
have realistic probabilities with this many molecules are ones in which the molecules
are approximately evenly distributed across the four regions. Thus, we can use a
microscopic (molecular level) model to demonstrate that the air in a room will not
spontaneously compress itself into one-quarter of the room’s volume. But we can
also analyze this same situation macroscopically, using principles discussed earlier
in this chapter. We learned through Example 4-4 that isothermally increasing the
volume of a gas increases its entropy; conversely isothermally compressing the gas
into a lower volume decreases its entropy. A gas can be compressed through a process that involves addition of work, such that the entropy of the system decreases
but the entropy of the universe increases. However, if the volume of gas in a system
spontaneously decreases, and nothing else happens, this process would violate the
second law of thermodynamics.
The preceding paragraph gives a simple example that illustrates the consistency
between the microscopic and macroscopic definitions of entropy. Mathematically,
the microscopic description of entropy is that entropy (S) of a system is proportional
to the number of microstates (Ω) corresponding to the macrostate that describes the
system:
S 5 kb ln V
(4.36)
The macroscopic definition of entropy described in Section 4.3.1 was proposed first.
Ludwig Boltzmann formulated the microscopic view of entropy (Sandler, 2011),
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C H A P T E R 4 Entropy
and demonstrated its mathematical consistency with the macroscopic definition.
Consequently, the constant of proportionality (kb) in Equation 4.36 is called
Boltzmann’s constant.
At the introductory level we will not explore how to use Equation 4.36 to compute changes in entropy for real systems and processes. The microscopic view of
entropy is introduced here to help build an understanding of entropy. It also lays the
foundation for measuring entropy on an absolute scale:
S 5 0 for a perfect crystal at absolute zero. This is sometimes called the third law of
thermodynamics.
A perfect crystal is one that has no defects—no impurities, no “missing atoms,” etc.
Why should such a crystal have zero entropy? Consider the 10 3 10 3 10 lattice pictured in Figure 4-13. If there are 1000 indistinguishable atoms in a crystal, then there
157
The Boltzmann
constant that appears
in equation 4.36 is
different from the
“Stefan-Boltzmann
constant,” which is
important in modeling
radiative heat transfer.
The equation PV 5
nRT was derived
empirically, from data,
before the kinetic
theory of the ideal gas
“proved” it. Similarly,
the macroscopic view
of entropy predates
the microscopic view,
but both are now understood to be consistent with each other.
Figure 4‑13 A simple crystal lattice containing 1000 atoms (10310310).
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158 Fundamentals of Chemical Engineering Thermodynamics
The fact that defects
in solids lead directly
to increased entropy is
likely another reason
why “measure of disorder” was at one time
a fashionable description of entropy.
is only one microstate—every lattice point is identical. Equation 4.36 reveals that if
the macrostate that describes the system contains only one microstate, then entropy
is zero (S 5 kb ln Ω 5 kb ln 1 5 0). However, if a macrostate has 999 indistinguishable
atoms and 1 defect, then there are 1000 microstates corresponding to 1000 different
locations in which the defect can be placed. More microstates mean greater entropy,
as indicated by Equation 4.36.
Next, you may ask—why must the crystal be at absolute zero to have zero entropy? Doesn’t the crystal in Figure 4-13 still have only one microstate, even if T 5 1 K,
or T 5 300 K? The thing to remember is that atoms and molecules are not in reality
stationary, even in a solid. While Figure 4-13 may accurately describe the location of
atoms in a crystal on average, it doesn’t capture the fact that the atoms are constantly
vibrating. Only at absolute zero, a hypothetical state where there is zero molecular
motion, could the simple picture shown in Figure 4-13 be valid all the time.
Because of the third law of thermodynamics, values of standard entropy in reference books can be expressed on an absolute scale (e.g., S 0) rather than related to a
reference state (e.g., ΔHf0). In an absolute sense, elements have positive internal energy, pressure, and volume and therefore have positive molar enthalpy (H 5 U 1 PV )
at any realistic conditions. But since we cannot measure energy on an absolute scale,
we establish a reference state in which H is considered zero for all elements at a standard set of conditions (conventionally T 5 258C and P 5 either 1 atm or 1 bar). The
“standard molar enthalpy of formation” of a compound, ΔH f0, is then defined as the
change in enthalpy that occurs when one mole of the compound is formed from its
elements at this standard set of conditions. However, the third law of thermodynamics allows us to measure entropy on an absolute scale. If the change in molar entropy
when a pure compound is isobarically heated from 0 K to 298.15 K (a process that
may or may not include phase changes) were 300 J/mol · K, that compound has S 0 5
300 J/mol · K in an absolute sense at 298.15 K, since it has zero entropy at 0 K. While
it isn’t realistic to make such a measurement experimentally, the theoretical models
we are building in this book can be used to quantify values for S 0.
4.4 the entropy balance
When we treat mass
and energy as conserved quantities, we
are ignoring nuclear
reactions that convert
mass into energy.
The mathematical definition of entropy, as given in Equation 4.14, is specified for a
closed, homogeneous system and is only directly applicable to reversible processes.
However, the examples in this book illustrate that engineers must be able to analyze
a wide variety of processes and systems, including both open and closed systems, and
including irreversible processes. Chapter 3 introduced generalized mass and energy
balances that are applicable to this multitude of systems. In this section we construct
the analogous entropy balance that it is applicable to any system.
As mentioned in Chapter 3, a balance equation can be written as
Accumulation 5 In 2 Out 1 Generation 2 Consumption
Throughout this book we are treating mass and energy as conserved quantities. Consequently, in our mass and energy balance equations (3.1 and 3.34), there is no “Generation” or “Consumption” term; our models need not account for the possibility
that mass or energy can be created or destroyed. By contrast Example 4-5 illustrates
that entropy is constantly being “created” by irreversible processes. Consequently,
·
the entropy balance includes a term Sgen that accounts for the generation of entropy
inside the boundaries of the system. It was noted in Example 4-4 that in a reversible process, the entropy of the universe is unchanged. The way this observation is
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C H A P T E R 4 Entropy
·
reflected mathematically is that in the entropy balance, the Sgen term is 0 for any
reversible process.
Regarding the “in” and “out” terms, entropy can be added to, or removed from,
a system through material flows. Entropy is also affected by heat transfer; if heat is
added to the system, then entropy is added to the system.
Thus, a general expression for an entropy balance equation is
·
j5J
k5K
n5N Q
d ( MSˆ )
·
n
·
·
mj,in Ŝj 2
mk,out Ŝk 1
5
1 Sgen
dt
T
k51
n51 n
j51
o
o
o
(4.37)
where
t is the time.
M is the mass of the system.
159
In the entropy
balance, Q represents the actual heat
transfer, not a reversible heat transfer as in
Equation 4.14.
Equations 4.37 and
4.38 assume that heat
transfer is occurring
at a system boundary
that has a constant
temperature. Use
Equation 4.39 for the
case of temperature
that changes with time.
Ŝ is the specific entropy of the system.
· and m
·
m
are the mass flow rates of individual streams entering and leaving
k,out
j,in
the system, and the summations are carried out over all such streams.
Ŝj and Ŝk are the specific entropies of streams entering and leaving the system.
·
Qn is the actual rate at which heat is added to or removed from the system at one
particular location, and the summation is carried out over all locations where heat
is added or removed.
Tn is the temperature of the system at the boundary where the heat transfer labeled
n occurs. Temperature must be expressed on an absolute scale (Kelvin or Rankine).
·
Sgen is the rate at which entropy is generated within the boundaries of the system.
In Chapter 3, the mass and energy balances were also expressed in time-independent
forms, for situations in which no information about the duration of the process was
known or needed.
The analogous time-independent entropy balance equation is
j5J
sM2 Ŝ2d 2 sM1 Ŝ1d 5
k5K
o m Ŝ 2 o m
j, in
j51
j
k51
k, out
Ŝk 1
n5N
Qn
n51
n
o T 1S
gen
(4.38)
where
FOOd FOr
thOught
4–12
If the entropy balance
is valid for ANY system, why do we also
need the “definition of
entropy” in Equation
4.14? What role can
Equation 4.14 play
in solving an entropy
balance?
mj,in and mk,out are individual quantities of mass added to or removed from the
process, and the summations are carried out over all such quantities.
Qn is an individual quantity of heat added to or removed from the system during
the process.
Sgen is the total entropy generated inside the boundaries of the system during the
process.
·
Any individual Q or Q can be either positive or negative. If heat is removed from a
system, then entropy is also removed from the system, and this phenomenon is re·
flected by the negative sign of Q or Q. Note that if the temperature Tn is not constant
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160 Fundamentals of Chemical Engineering Thermodynamics
with respect to time, then the heat added in each increment of time (dt) must be
treated as a separate heat transfer, and the summation becomes an integral:
·
Q in Equation 4.39
·
is the real Q for the
real process. It does
·
not have to be Q for a
reversible process.
sM2 Ŝ2d 2 sM1 Ŝ1d 5
j 5J
k5K
o m Ŝ 2 o m
j,in
j51
j
k,out
Ŝk 1
k51
#
·
tfinal Q
tinitial T
dt 1 Sgen
(4.39)
·
It should be emphasized that Q or Q, in any entropy balance, is the actual heat added
or removed. There is no requirement that the heat transfer be reversible, as there is in
the mathematical definition of entropy (Equation 4.14). The rationale for this apparent contradiction is as follows: As illustrated in Figure 4-7, heat transfer can be either
reversible or irreversible. The physical effect of the heat on the system (e.g., a temperature change and/or a phase change) is determined by the amount of heat added, and
is the same whether the heat transfer was reversible or irreversible. Thus, when we are
examining the process of heat transfer by itself, we can imagine the heat transfer was
reversible (even if it wasn’t) and apply Equation 4.14 to compute the change in entropy.
This problem-solving strategy was illustrated previously in Example 4-5.
·
Next we address the Sgen (or Sgen) term. If all other terms of Equations 4.37, 4.38,
·
or 4.39 can be evaluated, one can solve the entropy balance to determine Sgen, much
like we frequently solve an energy balance to determine Q or W. However, aside
·
from solving the entropy balance, there is no fundamental way to calculate Sgen. Con·
sequently, in this book Sgen either will be computed using the entropy balance or will
be set equal to 0 in processes that reasonably can be modeled as reversible.
The following sections provide several examples in which the entropy balance
is applied.
4.4.1 applying the entropy balance to a reversible nozzle
Example 4-6 illustrates the application of the entropy balance to a fairly simple
physical system, demonstrating the use of both data and the ideal gas model to solve
a problem. Subsequent examples involve more complex entropy balances.
examPle 4‑6
In Example 3-4, the
exiting temperature
was specified, but here
it is unknown. The entropy balance is an additional equation that
allows us to find this
additional unknown.
a reversibLe nOzzLe
A gas at T = 450°C and P = 5 bar flows at steady state through an adiabatic and reversible nozzle. The exiting pressure is 1 bar. Find the velocity and temperature of the exiting
stream if
A. The gas is steam.
B. The gas is an ideal gas with CV* = 3R and molecular weight = 75 g/mol.
Tin 5 4508C
Pin 5 5 bar
Tout 5 ?
Pout 5 1 bar
v5?
Figure 4‑14 Nozzle described in Example 4-6.
SOLUTION A:
Step 1 Define a system and write the energy balance
First, define the fluid in the nozzle as the system. The energy balance as listed in
Table 3-1 is
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C H A P T E R 4 Entropy
v2out
2
161
5 Ĥin 2 Ĥout
There is no information in the problem statement that isn’t consistent with the
assumptions behind this equation (adiabatic, potential energy negligible, entering
velocity negligible).
Step 2 Degree of freedom analysis
The specific enthalpy of the entering steam is found in the steam tables, Ĥin=
3377.7 kJ/kg. In Example 3-4, which also involved a nozzle, the energy balance could be
solved immediately because the outgoing temperature and pressure were both given.
However, an adiabatic nozzle has no mechanism for controlling temperature, so the current example, in which the exiting temperature and velocity are both unknown, is a more
realistic problem. With only one intensive property of the outlet stream known, we can’t
just look up Ĥout. The energy balance is thus one equation in two unknowns (Ĥout and vout).
The entropy balance provides the needed second equation.
The Gibbs phase rule,
introduced in Section
2.2.3, says that a single
component in a single
phase has two degrees
of freedom. Here only
one intensive property
of the outlet stream is
known.
Step 3 Apply and simplify the entropy balance
The entropy balance for this system simplifies greatly:
It is a steady-state process, so the accumulation term is 0.
·
It is adiabatic, so the Q∙T term is 0.
It is a reversible process, so there is no generation of entropy inside the system.
There is only one stream entering and only one stream leaving.
Consequently, the entropy balance (Equation 4.37) simplifies to
· Ŝ 2 m
· Ŝ
05m
in in
out out
At steady state, the entering and leaving mass flow rates must be equal to each other.
Dividing through by this mass flow rate gives
0 5 Ŝin 2 Ŝout
(4.40)
Step 4 Calculate terms in entropy balance
From Appendix A, at 450°C and P = 5 bar, Ŝin = 7.9465 kJ/kg · K. Equation 4.40 shows
that in this process the entropy of the steam is unchanged; thus Ŝout = 7.9465 kJ/kg · K.
From Appendix A, at P = 1 bar,
Ŝ = 7.8356 kJ/kg · K at 200°C
Ŝ = 8.0346 kJ/kg · K at 250°C
Notice that specific
entropy has the same
units as heat capacity.
Linear interpolation
is reviewed in Appendix B-1.
Interpolating between these values gives
Tout 5 2008C 1
1
kJ
kg ? K
kJ
8.0346 2 7.8356
kg ? K
7.9465 2 7.8356
2
s250 2 2008Cd 5 227.68C
Knowing the outlet temperature allows us to find Ĥout. Interpolating from the steam
tables gives
Ĥout 5 2875.5
1
kJ
227.68C 2 2008C
1
kg
2508C 2 2008C
2 12974.5 2 2875.5 kgkJ 2 5 2930.2 kgkJ (4.41)
For a pure compound
in a single phase, if
two intensive properties are known, all
others have unique
values. Pout was given.
Finding the second
property Tout allows us
to look up other properties in the steam
tables.
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162 Fundamentals of Chemical Engineering Thermodynamics
Step 5 Solve energy balance
Introducing this result into the energy balance gives
v2out
1
kJ
5 Ĥin 2 Ĥout 5 3377.7 2 2930.2
2
kg
21
vout 5 669.0
J
1000
kJ
21
N?m
1
J
21
kg ? m
s2
1
N
2
(4.42)
m
s
SOLUTION B:
Step 6 Compare/contrast with part A
Now we consider solution of the problem using the ideal gas model, rather than data.
The nozzle is adiabatic, reversible, and steady state, as it was in part A, so the energy and
entropy balances from steps 1 and 3 again apply:
v2out
2
5 Ĥin 2 Ĥout
0 5 Ŝin 2 Ŝout
For an ideal gas, enthalpy can be related to temperature through the heat capacity.
Thus, just as in part A, we can resolve the energy balance if we can determine Tout from
the entropy balance. However, unlike part A, there is no data available that relates
entropy to temperature.
Step 7 Apply definition of entropy
Besides using experimental data, how else can entropy and temperature be related to
each other? Through the definition of entropy:
dS 5
This use of the
concept of a state
property illustrates a
fundamental problemsolving strategy in
thermodynamics, as
discussed after the
example.
dQrev
T
which can be applied to any homogeneous, closed system. The nozzle is not a closed system, but we can utilize the fact that entropy is a state function.
We can imagine placing the ideal gas in a simple, closed system, like the pistoncylinder device from Example 4-3.
We can imagine that the gas in this closed system experiences the same change
in temperature and pressure as the gas travelling through the nozzle (T1 = Tin,
T2 = Tout , etc.).
We can imagine the changes in the closed system are reversible.
Because entropy is a state function, the change in S or Ŝ for a particular set of starting (T1, P1) and ending (T2 , P2) conditions is always the same, regardless of the path.
The energy balance
is converted into differential form because
we ultimately need to
integrate dQ/T
Step 8 Evaluate heat (dQrev ) for hypothetical, closed system
We know from Example 4-3 that, for our closed system, the energy balance is
U2 2 U1 5 WEC 1 Q
Or in differential form, it is
dU 5 dWEC 1 dQ
(4.43)
For any ideal gas process, U can be related to T through CV. Furthermore, we know dWEC
is given by –P dV.
dU 5 NCV dT 5 2P dV 1 dQ
(4.44)
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C H A P T E R 4 Entropy
163
We know from Section 1.4.3 that in this expression, P is the pressure opposing the motion.
When a gas contracts, the pressure of the gas itself is the pressure opposing the motion.
When a gas expands, the pressure opposing the motion is an external pressure.
But for a reversible process, the pressure opposing the motion and the pressure driving the motion must be essentially identical. Consequently, in Equation 4.44, P can
be the pressure of the ideal gas itself, regardless of whether the process involves
expansion or contraction.
Thus for any reversible ideal gas process occurring in a homogeneous, closed system,
dQrev 5 NC*V dT 1 P dV
(4.45)
Step 9 Evaluate change in entropy for hypothetical closed system
Substituting Equation 4.45 into the definition of entropy (Equation 4.14) gives
dS 5 N
CV*
T
dT 1
P
dV
T
(4.46)
Since this is a closed system, the number of moles (N) is constant. Thus, we can divide
through by N, which has the effect of converting S and V into the molar volume and the
molar entropy.
dS 5
CV*
T
dT 1
P
dV
T
The last term cannot be integrated unless P and T are expressed as functions of V, which
can be done through the ideal gas law: PV = RT. Thus,
dS 5
CV*
T
dT 1
R
dV
V
(4.47)
Step 10 Solve differential equation for change in entropy
For this example, CV* is a constant equal to 3R, so the integration of Equation 4.47 is
T2
S 2 2 S1 5 C*V ln
T1
1 R ln
V2
(4.48)
V1
The given information is in terms of T and P, not V. So it is convenient to substitute for
the molar volumes, again using the ideal gas law, for
1 2
RT2
S 2 2 S 1 5 C*V ln
1T 2 1 R ln RT
T2
1
P2
1
5 C*V ln
1T 2 1 R ln 1T P 2
T2
T2 P1
1
1
In steps 7 through 9,
we have assumed
nothing other than
ideal gas behavior.
Consequently, Equation 4.47 can be applied to any ideal gas
process, as discussed
further after the
example. In step 10,
the assumption is
that CV* is constant, so
subsequent equations
are not valid for ideal
gases in general.
(4.49)
2
P1
Equation 4.49 consists entirely of state properties (molar entropy, temperature, and
pressure), and is therefore path-independent; it can be applied to any ideal gas process
in which the heat capacity (CV*) is constant. Apply it to the nozzle by equating Tin = T1,
Pin = P1, Tout = T2, and Pout = P2 for
S out 2 S in 5 3R ln
1 T 2 1 R ln 1T P 2
Tout
in
Tout Pin
in
(4.50)
out
Step 11 Find exiting temperature
We determined in step 6 that S out – S in = 0. Combining this fact with Equation 4.50
gives
S and Ŝ differ from
each other only by a
multiplier, which is
the molecular weight.
Thus, if Ŝin 2 Ŝout 5 0,
then S in 2 S out 5 0.
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164 Fundamentals of Chemical Engineering Thermodynamics
PitFaLL
PreventiOn
In the definition of
entropy (equation
4.14), T must be expressed on an absolute
scale. Since Equation
4.51 is directly derived
from Equation 4.14,
the given temperature
must be converted
into Kelvin.
0 5 3R ln
1450 1 273.15 K2 1 R ln 3s450 1 273.15 Kds1 bard4
sToutds5 bard
Tout
(4.51)
The solution to equation 4.51 is Tout 5 483.60 K. This is close to the answer from part A,
which was 227.6°C or 499.8 K. We did not expect the answers to be identical; high pressure steam is not an ideal gas.
Step 12 Close energy balance
For any ideal gas process, dH = CP* dT. Here CP* is not given, but we can apply the ideal
gas property CP* = CV* + R, so CP* = 4R. Thus,
H out 2 H in 5
1
H out 2 H in 5 4 8.314
# C dT 5 4RsT 2 T d
Tout
Tin
*
P
out
(4.52)
in
2
J
J
s483.6 K 2 723.15 Kd 5 27966.5
mol ? K
mol
We complete the problem by inserting this value into the energy balance:
v2out
1
J
5 7966.5
2
mol
21
1 mol
75 g
21
1000 g
1k g
21
1N?m
1J
21
kg ? m
s2
1
N
2
(4.53)
The exiting velocity is vout = 460.9 m/s.
Steps 7 through 9 in Example 4-6 illustrate both a common problem-solving approach in thermodynamics and a useful property of ideal gases.
The problem-solving approach used the concept of a state property. The Gibbs
phase rule shows it takes two intensive state properties (e.g., T and P) to fully define
the state of a pure compound. Once the state is defined, all other intensive properties
(e.g., S) have unique values. Consequently, if one defines two states (T1, P1 and T2, P2),
one can evaluate the change in a state property (S in the example) along any path between these two states, and know that the result is valid for all other paths. This problemsolving approach is of particular importance for entropy because the definition of entropy
(Equation 4.14) only applies directly to reversible processes carried out in closed systems.
In steps 7–9 of Example 4-6, we modeled the change in entropy of an ideal gas.
Since no assumptions (beyond ideal gas behavior) were made in these steps, the
result is correct for ideal gases in general.
The change in molar entropy for any ideal gas is given by
dS 5
Equations 4.54 and
4.55 are valid for any
process involving ideal
gases.
CV*
T
dT 1
R
dV
V
(4.54)
One can carry out an analogous derivation on a mass basis and show that
dŜ 5
ĈV*
T
dT 1
R
V̂
(4.55)
dV̂
A common conceptual error is proceeding directly from Equation 4.54 to the integrated form of
S 2 2 S1 5 CV* ln
T2
T1
1 R ln
V2
V1
(4.56)
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C H A P T E R 4 Entropy
165
The integration of Equation 4.54 only yields Equation 4.56 when CV* is constant with
respect to temperature. If CV* is a function of temperature, as is generally true, a more
complex integration results. Example 4-8 examines such a case.
Another thematic problem-solving strategy illustrated by Example 4-6 is the
use of the ideal gas law to convert between pressure (P) and molar volume (V). For
example, substituting the ideal gas law into Equation 4.56 shows
1 2
RT2
S 2 2 S1 5 CV* ln
1T 2
T2
1 R ln
1
P2
RT1
(4.57)
P1
which can be rearranged using the property log(xy) = log(x) + log(y) as
S 2 2 S1 5 CV* ln
1T 2
T2
1 R ln
1
1T 2
T2
1 R ln
1
1P 2
P1
(4.58)
2
Since for an ideal gas, CP* = CV* + R, Equation 4.58 can be consolidated as
S 2 2 S1 5 C*P ln
1T 2 1 R ln 1P 2
T2
P1
1
2
(4.59)
Equation 4.59, like Equation 4.56, is only valid for ideal gas processes in which C
can be modeled as constant with respect to temperature.
*
P
Equation 4.59 also can
be written as
T2
S 2 2 S1 5 C*P ln
T1
1 2
2 R ln
1P 2
P2
1
4.4.2 entropy and efficiency
We have established that reversible processes are idealized processes that produce
the maximum work, or use the minimum work. This section introduces the use of an
efficiency to quantify how nearly real processes approximate these idealized, reversible processes. We begin with a closer look at the motivational example.
MOtivatiOnaL exaMpLe revisited
examPle 4‑7
Consider the turbine in part C of Example 4-1, in which steam entered at T = 550°C and
P = 30 bar and exited at T = 200°C and P = 1 bar.
A. How much entropy is generated for each kilogram of steam that goes through this
turbine?
B. What is the maximum work that can be obtained (per kilogram of entering steam)
from a turbine with this inlet stream and an outlet pressure P = 1 bar?
SOLUTION A:
Step 1 Define a system and write an energy balance
As in Example 4-1, the energy balance for the turbine is
·
WS
· 5 Ĥout 2 Ĥin
m
Step 2 Apply and simplify entropy balance
To find the rate of entropy generation, an entropy balance is the logical way to begin.
Starting with the time-dependent entropy balance (Equation 4.37):
·
j 5J
k5K
n5N Q
d sMŜd
·
n
· Ŝ 2
·
m
m
1
5
Ŝ
1 Sgen
j,in j
k,out k
dt
T
k51
n51
n
j51
o
o
o
This problem can
be solved starting
from either the timedependent or the
time-independent
balance, since we seek
answers on a basis
of “per kilogram of
steam.” If we wanted
to find the rate of
entropy generation
(e.g., in kJ/K · s) we
would definitely use
the time-dependent
equation.
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166 Fundamentals of Chemical Engineering Thermodynamics
Like the nozzle in Example 4-6, this is an adiabatic, steady-state process, so the accumula·
tion and rate of heat transfer (Q) terms are 0. Consequently the entropy balance is
· Ŝ 2 m
· Ŝ 1 S·
05m
(4.60)
in in
out out
gen
For this steady-state process the entering and exiting mass flowrates are identical
· 5m
· 5m
· d. Solving for the generation of entropy:
sm
in
out
·
Sgen
(4.61)
· 5 sŜout 2 Ŝind
m
You can show that
for the other possible
outlet streams listed in
the Motivational
Example, the value
·
of Sgen calculated
in Step 3 would be
negative.
As in Example 4-6,
the energy balance
here has two
unknowns, and the
entropy balance is a
second equation.
The left-hand side is now exactly what we wish to find: the entropy generated per kilogram of steam.
Step 3 Introduce data
Because temperature and pressure are both known for the entering and exiting streams,
entropy of each can be found in the steam tables.
·
Sgen
kJ
kJ
(4.62)
· 5 7.8356 2 7.3768 kg ? K 5 0.4588 kg ? K
m
1
2
SOLUTION B:
Step 4 Apply energy balance
As in part A, the energy balance is
·
WS
· 5 Ĥout 2 Ĥin
m
In part A, the conditions of the outlet stream were exactly known, so Ĥout could be
obtained immediately from the steam tables. Here, only pressure is known for the outlet
stream—we don’t know the outlet temperature that corresponds to “maximum work.”
We need a second property of the outlet stream before we can look up Ĥout, and we can
find it by applying the concept of the reversible process.
Step 5 Entropy balance for reversible turbine
As illustrated in Examples 4-2 and 4-3, the maximum work is obtained from a reversible
process. Thus, in part B, we are analyzing a reversible turbine that has an outlet pressure
of 1 bar. As stated in Section 4.3.1, no entropy is generated in a reversible process. Consequently the entropy balance (equation 4.61) simplifies to:
0 5 sŜout 2 Ŝin d
(4.63)
Since the inlet stream is the same as it was in part A, for the reversible turbine:
Ŝin 5 Ŝout 5 7.3768
kJ
kg ? K
(4.64)
Step 6 Find outlet temperature
According to the steam tables, at P = 1 bar, Ŝ= 7.3614 kJ/kg · K for steam at 100°C and
Ŝ 5 7.6134 kJ/kg · K for steam at 150°C. Interpolating between these two values gives:
Tout 5 1008C 1 s508Cd
1
kJ
kg ? K
kJ
7.6134 2 7.3614
kg ? K
7.3768 2 7.3614
2
5 103.18C
(4.65)
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C H A P T E R 4 Entropy
167
Step 7 Find specific enthalpy of outlet stream
Interpolating a second time to find the specific enthalpy of steam at P 5 1 bar and
T 5 103.1°C:
Ĥout 5 Ĥ100 1
Ĥout 5 2676.2
1
1 150 2 1008C 2 sĤ 5 Ĥ d
103.1 2 1008C
103.1 2 1008C
kJ
1
kg
150 2 1008C
150
100
(4.66)
2 12776.4 2 2676.2 kgkJ 2 5 2682.3 kgkJ
Step 8 Close energy balance
The maximum work, which is equivalent to the reversible work, is found by inserting
the result of part 7 into the energy balance:
·
WS
kJ
(4.67)
· 5 Ĥout 2 Ĥin 5 2682.3 2 3569.7 5 2887.4 kg
m
Example 4-7 mirrors the motivational example given in Section 4-1. The motivational example described four possible sets of outlet conditions for a steady-state
turbine and asserted that three of them were physically impossible, because the
entropies of the possible outlet streams were smaller than the entropy of the inlet
stream. The entropy balances presented in steps 1 through 3 of Example 4-7 illustrate the same principle in a more formal mathematical way; the process is physically
·
impossible if the calculated Sgen is negative.
Example 4-7 also illustrates the application of the principle of reversibility to
calculate maximum work. A reversible process is a hypothetical limiting case that
represents the maximum work produced, or the minimum work required. We can
define an efficiency that compares the work produced by a real turbine to the theoretical maximum represented by a reversible process, as
·
WS, actual
turbine 5 ·
(4.68)
WS, reversible
For the turbine in Example 4-7, the real work is 2694.2 kJ/kg and the reversible work
is 2887.4 kJ/kg, so the efficiency is 0.782 or 78.2%.
A similar definition can be applied to a pump or compressor:
·
WS, reversible
(4.69)
compressor 5 ·
WS, actual
Our sign convention
is that work done by
the system on the surroundings is negative
(energy leaving the
system). Some books
use the opposite sign
convention. Reversible
work and actual work
will have the same
sign as each other, so
efficiency is positive
regardless of the sign
convention used.
Note that a compressor is a process that requires work, and the reversible compressor represents the minimum work. Thus, for a compressor, the reversible work is
smaller than the actual work. Equations 4.68 and 4.69 are both defined with the
larger quantity in the denominator, so that efficiency will always be a fraction (0 2 1
or 0 2 100%). Equation 4.69 is applied in the next example.
cOMPressiOn OF an ideaL gas
examPle 4‑8
Ammonia enters an adiabatic, steady-state compressor at P = 0.2 bar and T = 300 K
and leaves at P = 1 bar (Figure 4-15). Ammonia can be assumed to act as an ideal gas at
pressures up to 1 atm. The compressor efficiency is 75%. Find:
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168 Fundamentals of Chemical Engineering Thermodynamics
P 5 0.2 bar
T 5 300 K
5 0.75
P 5 1.0 bar
T5?
Ws 5 ?
Figure 4‑15 The compression
of ammonia, which is examined in
Example 4-8.
A. The work required for each mole of gas compressed.
A compressor is an
integral component
of a refrigerator––the
compressor is what
actually makes the
noise you hear when
the refrigerator
mechanism is operating. Refrigeration is
discussed in detail in
Section 5-3.
You can prove the
equivalence of
Equations 4.70 and
4.71 rigorously by
applying the fact that
H 5 NH 5 MĤ.
At the time of writing,
a Wikipedia data page
(http://en.wikipedia.
org/wiki/Ammonia_
(data_page) said
gaseous ammonia had
CP = 35.06 J/mol · K at
0°C. What if we used
a simpler approximation and assumed
CP* = 35.06 J/mol · K
was a constant value
and correct at all
temperatures?
If we were assuming
CP* was constant, the
integral in Equation 4.73 would be
H out2 H in5C *P sTout2Tind.
B. The temperature of the outgoing gas.
SOLUTION:
Step 1 Define a system
The compressor is the system.
Step 2 Apply and simplify the energy balance
Section 3.6.3 presented the energy balance for an adiabatic, steady-state compressor:
·
WS
(4.70)
· 5 Ĥout 2 Ĥin
m
Here the given information is on a molar basis, not a mass basis, so the equation is better
written as
·
WS
(4.71)
5 H out 2 H in
n·
Step 3 Relate enthalpy to temperature
It was established in Section 2.3.3 that, for an ideal gas,
dH 5 CP* dT
Appendix D gives ideal gas heat-capacity data for various compounds in the form of
C*P 5 A 1 BT 1 CT 2 1 DT 3 1 ET 4
(4.72)
Thus, the change in molar enthalpy is given by
H out 2 H in 5
# A 1 BT 1 CT 1 DT 1 ET dT
Tout
2
3
4
(4.73)
Tin
Tin = 300 K, but Tout is unknown, which prevents us from solving the problem
immediately.
Step 4 Apply entropy balance to compressor
As in Example 4-7, the entropy balance for an adiabatic, steady-state compressor simplifies to
·
0 5 n· S 2 n· S 1 S
(4.74)
in
in
out
out
gen
The entering and exiting flow rates are identical in this steady-state process:
·
0 5 n· sS 2 S d 1 S
in
out
gen
(4.75)
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169
C H A P T E R 4 Entropy
·
Here, Sgen is an unknown that makes Equation 4.75 impossible to solve directly for the
real turbine.
Step 5 Apply entropy balance to a reversible compressor
We now consider a hypothetical reversible compressor, with the same inlet stream and
·
outlet pressure as the actual compressor. Since Sgen is zero in a reversible process, the
entropy balance is now
0 5 n· sS in 2 S out,revd
(4.76)
0 5 sS in 2 S out,revd
(4.77)
If we were assuming
CP* was constant at
35.06 J/mol · K, then
we would say CV* =
CP* – R = 35.06 –
8.314 = 26.75 J/mol · K.
Step 6 Relate entropy change to temperature
Equation 4.54 is valid for any ideal gas process:
dS 5
CV*
T
dT 1
PitFaLL
PreventiOn
R
dV
V
CV* isn’t known directly, but CP* is known, and we can apply the property of ideal gases
CV* + R = CP* for
C*V 5 sA 2 Rd 1 BT 1 CT 2 1 DT 3 1 ET 4
(4.78)
Step 7 Integrate differential equation to find outlet temperature
Applying Equation 4.54 to the reversible compressor yields
S out,rev 2 S in 5
#
Tout,rev sA 2 Rd 1 BT 1 CT 2 1 DT 3 1 ET 4
T
Tin
dT 1
R
dV
V
(4.79)
If we were assuming
CP* was constant,
Equation 4.80 would be
Tout,rev
C
1 BsTout,rev 2 Tin d 1 sT 2out,rev 2 T 2ind
S out,rev 2 S in 5 sA 2 Rd ln
Tin
2
V out,rev
E
D
1 sT 3out,rev 2 T 3in d 1 sT 4out,rev 2 T 4in d 1 R ln
3
4
V in
0 5 sC*P 2 Rd ln
We showed in step 5 that the entropy change for the reversible process is 0. In addition,
since the given information is in terms of P and T, rather than V, we can substitute for V
using the ideal gas law for
0 5 sA 2 Rd ln
Tout, rev
Tin
C
1 BsTout,rev 2 Tin d 1 sT 2out, rev 2 T 2ind
2
The temperature
found in step 7 is not
the answer to part B.
Here we are calculating the temperature
leaving a hypothetical
reversible compressor, NOT the real
compressor.
Tout,rev
Tin
RTout,rev
1 R ln
Pout
RTin
Pin
(4.80)
and its solution would
be Tout,rev = 444.4 K.
RTout,rev
1
Pout
D 3
E
sT out, rev 2 T 3in d 1 sT 4out, rev 2 T 4in d 1 R ln
RT
3
4
in
Pin
The parameters A, B, C, D, and E are available in Appendix D. For ammonia,
A = 25.235, B = 235.044 3 10–3, C = 16.969 3 10–5, D = 217.676 3 10–8, and E =
6.327 3 10–11 for T expressed in K and CP* expressed in J/mol · K. Consequently, while this
is a messy equation, the only unknown in it is the outlet temperature from the reversible
turbine, Tout,rev. It can be solved to determine Tout,rev = 428.7 K.
There is no easy
analytical solution
to Equation 4.80,
but many modern
calculators can be
used to solve such
equations numerically,
as can software like
Excel, MATLAB, and
POLYMATH.
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170 Fundamentals of Chemical Engineering Thermodynamics
If we were assuming
CP* was constant, this
equation would be
·
WS,rev
5 H out 2 H in
n·
5 CP* sTout, rev 2 Tin d
and the answer would be
·
WS, rev
J
5 5063
n·
mol
In computing efficiency,
work can be expressed
in absolute terms (WS),
·
as a rate (WS), or ·on a
W
mass or molar · S
n
1 2
basis. But reversible
and actual work must
be expressed on the
same basis.
If we were assuming
Cp* was constant,
this would be
J
·
WS,actual 5063 mol
5
n·
0.75
J
5 6750
,
mol
which is ~5% larger
than the real answer.
If we were assuming
CP* was constant, this
would be
·
WS, act
5 CP*sTout, act 2 Tind
n·
and the solution would
be Tout,act =492.5 K.
PitFaLL
PreventiOn
Efficiencies of compressors, pumps, and
turbines cannot be
used to calculate temperatures directly––
they can only be
applied directly to
work, according to the
definitions in Equations 4.68 and 4.69.
Step 8 Find reversible work
The energy balance in Equation 4.71 can now be solved:
·
WS, rev
5 H out 2 H in 5
n·
#
T5428.7 K
A 1 BT 1 CT 2 1 DT 3 1 ET 4 dT
(4.81)
T5300 K
·
WS, rev
B 2
C 3
2
3
·n 5 AsTout,rev 2 Tin d 1 2 sT out,rev 2 T in d 1 3 sT out,rev 2 T in d
1
D 4
E
sT
2 T 4in d 1 sT 5out,rev 2 T 5in d
4 out,rev
5
·
WS, rev
J
5 4839
n·
mol
Step 9 Apply efficiency
The definition of efficiency for a compressor is
·
WS, reversible
compressor 5 ·
WS, actual
Here the efficiency is 0.75, so
4839
0.75 5
J
mol
·
WS,actual
n·
(4.82)
·
WS,actual
J
5 6452
n·
mol
Step 10 Determine actual outlet temperature
In step 8, we knew the outlet temperature for the reversible turbine and used Equation
4.81 to find the reversible work. Here we are faced essentially with the inverse problem:
we know the actual work, and wish to find the actual outlet temperature. The integration
in Equation 4.81, however, is still valid:
·
WS, act
B
C
(4.83)
5 AsTout, act 2 Tin d 1 sT 2out, act 2 T 2in d 1 sT 3out, act 2 T 3ind
n·
2
3
1
D 4
E
sT out, rev 2 T 4in d 1 sT 5out, rev 2 T 5in d
4
5
The actual outlet temperature is 468.8 K.
4.4.3 unsteady-state entropy balances
Examples 4-6 through 4-8 examined steady-state, adiabatic systems. Consequently, the entropy balance equations were comparatively simple. This section
examines the application of the entropy balance to systems that are not at steady
state.
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C H A P T E R 4 Entropy
cOOLing OF a sOLid
A solid sculpture (Figure 4-16) with M = 2 lbm and ĈP = 0.5 BTU/lbm · °R is heat-cured in
an oven at 250°F and ambient pressure. It is removed from the oven, placed on a table,
and allowed to cool to room temperature, which is 70°F. The room is large enough that
the heat released by the sculpture has no significant effect on the room temperature. Find
the amount of heat released, the change in entropy of the sculpture, and the change in
entropy of the universe.
Q5?
M 5 2 lbm
M 5 2 lbm
T 5 2508F
T 5 708F
Ĉ P 5 0.5 BTU/lbm8R
Ĉ P 5 0.5 BTU/lbm8R
171
examPle 4‑9
FOOd FOr
thOught
4-13
250°F is above the
boiling point of water.
If you heat a piece of
fresh pottery to 250°F,
would you expect to
remove all of the moisture from the pottery?
1 BTU, or British
thermal unit, is the
amount of energy
required to heat 1 lbm
of water from 39 to
40°F.
Figure 4‑16 Cooling process examined in Example 4-9.
SOLUTION:
Step 1 Define the system and process
First, define the sculpture as the system. The process begins when the sculpture is first placed
on the table (T1 = 250°F) and ends when the sculpture reaches room temperature (T2 = 70°F).
Step 2 Simplifying assumption
The surface of the object will reach room temperature sooner than the inside. However,
we will simplify the process by modeling the object as at a uniform temperature.
Step 3 Apply and simplify entropy balance
There is no information concerning rate or duration of the process, so we use the timeindependent entropy balance. The solid object is a closed system, so our first inclination
might be to simplify the time-independent entropy balance (Equation 4.38) to
sM2Ŝ2d 2 sM1Ŝ1d 5
Q
1 Sgen
Tsculpture
(4.84)
The left-hand side of the equation (the accumulation term) represents the change in
entropy of the system, which is what we were asked to determine. Q can be computed
using an energy balance; however we must recognize that Tsculpture, the temperature of the
system, is changing throughout the process. Consequently we begin with Equation 4.39
rather than Equation 4.37, and the entropy balance is
S2 2 S1 5
#T
dQ
1 Sgen
(4.85)
sculpture
The integral reflects that each increment of heat is added at a different temperature, and
these increments all need to be summed.
Step 4 Consider generation of entropy
A likely source of confusion concerns the Sgen term. We know that Sgen = 0 in reversible
processes. This is clearly not a reversible process; there is initially a 180°F temperature
FOOd FOr
thOught
4-14
Is this assumption
actually necessary to
solve the problem?
A typical chemical
engineering curriculum includes at least
one course on heat
transfer, covering the
principles needed
to model the nonuniform temperature
of an object.
By definition S 5 MŜ
pitFaLL
preventiOn
T in the entropy balance is the temperature of the system at
the boundary where
the heat transfer takes
place.
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172 Fundamentals of Chemical Engineering Thermodynamics
A process that has
irreversible heat
transfer at the system
boundary, but no other
irreversible feature,
is called “internally
reversible.” By our
definition Sgen = 0 for
reversible and also for
internally reversible
processes.
difference between the sculpture and the surroundings. The crucial insight is that Sgen
represents entropy generated inside the boundaries of the system. The irreversible feature of the process is heat transfer, which occurs at the boundaries of the system. The
system is a solid object that is being modeled as constant temperature, and thus there
is no particular process occurring inside the system that would generate additional
entropy.
Consequently, though the process overall is not reversible, we assume Sgen = 0. Equation 4.85 becomes:
S2 2 S 1 5
#T
dQ
(4.86)
sculpture
As in Example 4-5,
this example involves
irreversible heat
transfer. We again
note that the physical
effect of heat transfer
(here, lowering of
the temperature of
the sculpture) is the
same whether the heat
transfer is reversible
or irreversible.
When we compute the change in entropy of the universe we will see that the irreversible heat transfer is indeed accounted for, even with Sgen set equal to 0.
It is not possible to make further progress with Equation 4.86 unless Q and T are
related to each other.
Step 5 Apply and simplify energy balance
The sculpture is stationary, so there is no accumulation of kinetic or potential energy.
A solid object with no moving parts would not produce shaft work. Consequently the
energy balance simplifies to:
U2 2 U1 5 Q 1 WEC
(4.87)
Step 6 Evaluate work
As always dWEC = 2P dV. We don’t know whether the volume of the object changes
significantly as T changes, but we do know the object and room are at constant, ambient
pressure, so WEC integrates to:
WEC 5 2PsV2 2 V1d
(4.88)
Step 7 Quantify heat
Introducing the result of step 6 into the energy balance and solving for Q gives
Q 5 sU2 2 U1d 1 PsV2 2 V1d
(4.89)
Since H = U + PV, we can equate the right-hand side to
Q 5 sH2 2 H1d
PitFaLL
PreventiOn
A common conceptual error at this point
is inserting the value
Q = 2180 BTU
directly into the Q∙T
term of the entropy
balance, forgetting
that T isn’t constant.
(4.90)
Because this is a constant-pressure process, we know dH = MCp dT. Here ĈP is a
constant:
Q 5 sH2 2 H1d 5
# MĈ dT
T2
T1
P
(4.91)
1 lbBTU8R2 s70 2 2508F d 5 2180 BTU
Q 5 MĈP sT2 2 T1d 5 s2 lbmd 0.5
m
Step 8 Find entropy change of sculpture
Since T isn’t constant, we need a relationship between Q and T that would allow us to
integrate Equation 4.86. Thus, we can take the derivative of Equation 4.91, producing:
dQ 5 dH 5 MĈP dT
(4.92)
Inserting expression (4.92) into expression (4.86) gives:
S2 2 S1 5
#
T2 MĈP
T1
T
dT
(4.93)
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C H A P T E R 4 Entropy
S2 2 S1 5 MĈP ln
1T 2 5 s2 lb d 10.5 lb ? 8R2 ln 1250 1 459.678R2
T2
BTU
70 1 459.678R
pitFaLL
preventiOn
m
1
173
m
BTU
8R
This value represents the change in entropy of the sculpture. Recall that it is physically
impossible for the change in entropy of the universe to be negative, or for entropy generation (Sgen) to be negative, but there is nothing physically unrealistic about a system
experiencing a negative change in entropy.
Temperature must
always be expressed
on an absolute scale
when using the entropy balance. Here
the Rankine scale is
used.
Step 9 Apply entropy balance to room
We were also asked to calculate the change in entropy of the universe. Aside from the
sculpture, the only other portion of the universe affected by the process is the room, so
we now define a new system as “the entire room excluding the sculpture.” For this system,
there is no mass entering or leaving the system, and this time, the temperature of the
system is constant, so the entropy balance is
T (°R) = T (°F)
+ 459.67
S2 2 S1 5 20.293
S2.room 2 S1,room 5
Q
1 Sgen
Troom
(4.94)
As we did when we were analyzing the sculpture, we recognize that the heat transfer is
irreversible, but because the sculpture is not part of the system, the heat transfer occurs
at the system boundary. We again assume that no entropy is generated inside the system
boundaries and that Sgen is again 0.
Step 10 Calculate change in entropy of universe
When the room is the system, the total heat transferred is +180 BTU, since the heat given
off by the sculpture was added to the room.
S2, room 2 S1, room 5
Q
180 BTU
BTU
5
5 0.340
Troom 70 1 459.67 8R
8R
(4.95)
Thus, the change in entropy of just the room is 0.340 BTU/°R. The change in entropy of
the universe is ΔSroom+ΔSsculpture, or 0.340 – 0.293 = 0.047 BTU/°R.
You can also calculate
change in entropy of
the room by noting
the room acts as a heat
reservoir and applying
Equation 4.15.
FOOd FOr
thOught
4-15
Entropy has been
described as a metric
for lost work. This
process increases entropy of the universe,
so it must somehow
involve lost work.
What’s the “lost
work” here?
Example 4-9 shows what some might consider an odd, counter-intuitive result: When
we analyzed the system, we set Sgen = 0, and when we analyzed the surroundings, we
again set Sgen = 0, yet the final result was a positive change in entropy of the universe,
showing that entropy was indeed generated. The entropy generation occurred at the
boundary between the system and the surroundings, in the form of an irreversible
heat transfer. Sgen represents entropy generated inside the boundaries of the system.
The next example re-visits Example 3-11, in which a hot liquid is continuously
mixed into a cooler liquid. It is both an open system and an unsteady-state process.
an unsteady-state, Open systeM
examPle 4‑10
Recall the 500-liter adiabatic tank that was analyzed in Example 3-11. It was full of liquid
initially at T = 300 K. Valves were opened, allowing 20 kg/min of liquid at T = 400 K
to enter while 20 kg/min of liquid was also removed, as illustrated in Figure 4-17. After
5 minutes, the liquid in the tank had increased in temperature to 322.1 K. How much
entropy is generated within the tank during these 5 minutes?
SOLUTION:
Step 1 Define the system
We define the tank as the system, since our goal is to find the entropy generation in the tank.
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174 Fundamentals of Chemical Engineering Thermodynamics
Flow Rate 5 20 kg/min
Tin 5 400 K
V 5 500 L
T0 5 300 K
r 5 .8 kg/L
Ĉ v 5 3 J/g-K
21)t]
Flow Rate 5 20 kg/min
Tout 5 T
T 5 400 K 2 (100 K)e [2(0.05 min
Figure 4‑17 Schematic of a tank with liquid continuously entering and exiting. The
temperature of the tank, as a function of time, was determined in Example 3-11.
PitFaLL
PreventiOn
dsMŜd is the change
in the total entropy of
·
the system, and Sgen
is the rate at which
entropy is generated in
the system. These are
related in that entropy
generation is one of
the phenomena that
affect the total system
entropy, but they are
two different things.
·
Sgen is a rate of entropy
generation. To find
the total quantity of
entropy generated,
we must integrate
with respect to dt. The
total, Sgen, is what we
are actually asked to
calculate.
Step 2 Apply and simplify the entropy balance
Accounting for time is clearly important in part B, so we start with the time-dependent
entropy balance:
·
j5J
n5N Q
k5K
dsMŜd
·
n
·
·
mj, inŜj 2
(4.96)
5
1 Sgen
mk, out Ŝk 1
dt
Tn
n51
k51
j51
o
o
o
This is an adiabatic process, and there is only one stream entering and one stream leaving,
so the entropy balance simplifies to
dsMŜd
· Ŝ 1 S·
· Ŝ 2 m
5m
out out
in in
gen
dt
(4.97)
Three further simplifications can be noted:
· =m
· =m
· ).
1. The entering mass flow rate is equal to the exiting mass flow rate (m
in
out
2. The mass of the system (M) is constant, and can be pulled out of the differential.
3. The system is well mixed, so the properties of the exiting stream are identical to the
properties of the liquid in the tank. This means Ŝ, which is the specific entropy of
the liquid in the tank, is identical to Ŝout, which is the specific entropy of the exiting
liquid.
Applying these simplifications gives
M
dsŜd
· sŜ 2 Ŝd 1 S·
5m
in
gen
dt
(4.98)
Step 3 Define Sgen
We were asked to determine the quantity of entropy, Sgen, that is generated in 5 minutes.
This quantity is mathematically defined as:
Sgen 5
#
t55 min ·
t50
Sgen dt
(4.99)
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 4 Entropy
Step 4 Evaluate differential equation
Multiplying Equation 4.98 through by dt and integrating allows us to introduce Sgen, as
defined in step 3.
· sŜ 2 Ŝddt 1 S· dt
M dŜ 5 m
in
gen
MsŜfinal 2 Ŝinitiald 5
#
t55 min
t50
· sŜ 2 Ŝd dt 1 S
m
in
gen
(4.100)
Notice the left-hand side was easy to integrate, but Ŝfinal 2 Ŝinitial is not yet known. The
· sŜ 2 Ŝd term can’t currently be integrated with respect to dt, because we don’t have a
m
in
mathematical relationship between Ŝ and t. From Example 3-11, we do know a relationship between temperature (T) of the tank and time (t). What we still need is a relationship between Ŝ and T.
Step 5 Construct a hypothetical reversible process for computing entropy
Consider the quantity Ŝfinal 2 Ŝinitial; we know that the system contains 400 kg of liquid,
that the initial system temperature is T = 300 K, and the final system temperature is
T = 322.1 K. If we imagine that the 400 kg of liquid was a closed, homogeneous system
and that it was heated reversibly from T = 300 K to T = 322.1 K, we can find Qrev and
apply the definition of entropy (Equation 4.14).
Step 6 Apply and simplify energy balance to closed, reversible system
The energy balance for a closed system with no shaft work can be written as
dU 5 dQrev 1 dWEC
(4.101)
Ŝin is a constant,
because the entering
stream has a constant
T = 400 K. However,
Ŝ is not constant; it
represents the specific
entropy of the liquid
in the tank, which is
changing as the tank
temperature changes.
FOOd FOr
thOught
4-16
Why are we using a
closed system in this
step, when the tank is
obviously an open system and the entropy
balance in step 2 was
for an open system?
The density of the liquid is constant (0.8 kg/L, as given in Example 3-11). This means the
system (liquid) volume V will remain constant as the temperature changes. This fact is
significant in two respects: first, there is no work of expansion/contraction, and second,
dU can be equated to MCV dT:
MĈV dT 5 dQrev
(4.102)
Step 7 Apply definition of entropy
Introducing Equation 4.102 into the definition of entropy:
dS 5
dQrev
T
5
MĈV dT
(4.103)
T
We divide through by M, since Equation 4.100, which is what we are ultimately trying to
solve, is written in terms of specific entropy:
dŜ 5
ĈV dT
(4.104)
T
Step 8 Evaluate Ŝfinal 2 Ŝinitial
Physically, Ŝfinal 2 Ŝinitial represents the change in specific entropy as the liquid increases
temperature from Tinitial = 300 K to Tfinal = 322.1 K, which can be found by integrating
Equation 4.104 for
Ŝfinal 2 Ŝinitial 5
#
T5322.1 K ĈV dT
T5300 K
T
5 ĈV ln
1 300 K 2
322.1K
(4.105)
1 kg ? K2 ln 1 300 K 2 5 0.213 kg ? K
Ŝfinal 2 Ŝinitial 5 3
kJ
322.1 K
175
kJ
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
176 Fundamentals of Chemical Engineering Thermodynamics
Step 9 Relate Ŝin 2 Ŝ to t
The entropy balance also includes the integral of (Ŝin 2 Ŝ) dt, which represents the difference in specific entropy between the entering and exiting streams. The entering stream is
at a constant temperature of T = 400 K, but we cannot assign a specific numerical value
to the exiting temperature, because it changes with time. Integrating Equation 4.104 from
the variable outlet temperature T to T = 400 K:
Ŝin 2 Ŝ 5
#
T5400K ĈV dT
T
T5T
5 ĈV ln
1 T 2
400 K
(4.106)
We now substitute the known relationship between T and t into Equation 4.106, so that
we can integrate with respect to dt:
K
1400T K2 5 Ĉ ln 1400 K 400
2
2 100 e
Ŝin 2 Ŝ 5 ĈV ln
The mass flow rate m·
and heat capacity ĈV
are both constant with
respect to t and can
be pulled out of the
integral.
(4.107)
20.05t
Step 10 Solve entropy balance
We can now evaluate Equation 4.100
MsŜfinal 2 Ŝinitiald 5
MsŜfinal 2 Ŝinitiald 5
1
s400 kgd 0.213
The integral was evaluated using POLYMATH, a software
package that includes
a numerical ODE
solver.
V
2 1
#
t55 min
t50
kg
kJ
5 20
kg ? K
min
#
t55 min
t50
· C ln
m
V
· sŜ 2 Ŝd dt 1 S
m
in
gen
3400 K 2400s100K Kde 4 dt 1 S
20.05t
2 13 kg ? K2#
kJ
t55 min
t50
ln
gen
(4.108)
3400 K 2 s100 Kde 4 dt 1 S
400 K
20.05t
gen
The integral can be evaluated numerically, and is equal to 1.251 min
1
s400 kgd 0.213
2 1
kg
kJ
5 20
kg ? K
min
2 13 kgkJ? K2 s1.251 min d 1 S
Sgen 5 10.14
gen
(4.109)
kJ
K
Example 4-10 shows that when hot and cool liquid are mixed together,
entropy is generated, and it shows how the entropy generation can be quantified.
Example 4-9 showed that entropy is generated when a hot object cools. This chapter has proposed that entropy can be regarded as a metric for lost work. Since
entropy is being generated in Example 4-9 and Example 4-10, these examples must
involve “lost work” in some form. Where is the “lost work” in these heat transfer
processes?
The motivational examples in Chapters 1 and 2 presented the Rankine
cycle as an important system that illustrates the significance of several different thermodynamics concepts. These sections illustrated a way to convert heat
into work. Thus, any hot substance has some potential to do work. Example 4-10
involved mixing liquid at 400 K into a tank of cooler liquid. As the liquids
mixed, the entering liquid cooled, giving it less potential to do work, while the
liquid in the tank warmed, giving it more potential to do work. Thus at first glance
it is unclear that a net loss of potential work has occurred. The next section demonstrates how “potential to do work” is inherently related to temperature.
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C H A P T E R 4 Entropy
177
4.5 the carnot heat engine
Throughout this chapter, we have considered systems that were single items: single
pieces of equipment, a cylinder containing a gas, a sculpture, etc. Like the energy
balance, the entropy balance can also be applied to larger systems, such as complete processes. Here, we consider application of the entropy balance to a heat
engine. We define a heat engine as a process that is designed to convert heat into
work.
The Rankine cycle
described in Sections
1.2.3 and 2.1 is a heat
engine, but it is not the
only possible design of
a heat engine.
a reversibLe heat engine
examPle 4‑11
This problem investigates a heat engine that operates as shown in Figure 4-18.
Hot reservoir: TH 5 800 K
QH 5 1000 kJ/min
Heat Engine
W5?
Many processes
involve two distinct
heat reservoirs: one
for heating and one
for cooling. We use TH
to represent the temperature of the “hot”
reservoir and TC for
the temperature of the
“cold” reservoir. These
symbols are used extensively in Chapter 5.
QC 5 ?
Cold reservoir: TC 5 300 K
Figure 4‑18 Heat engine described in
Example 4-11.
The engine is a closed system that operates continuously at a steady state.
1000 kJ/min of heat are added to the engine from a high-temperature reservoir at
TH = 800 K.
Heat is emitted from the engine to a low-temperature reservoir at TC = 300 K.
Shaft work is produced.
Assuming that the entire process is reversible, determine the rate at which work is produced and the rate at which heat is emitted.
SOLUTION: Before beginning, we note that we know nothing specific about the unit
operations used in the heat engine. We know only that the process is reversible and operating at a steady state. We will see here that these facts are sufficient to solve the problem.
·
There are two unknowns, the shaft work produced, WS, and the heat emitted to the low·
temperature reservoir, which we will call QC . Consequently, we need two equations: an
energy balance and an entropy balance.
Step 1 Define a system
The system is the entire heat engine, as shown in Figure 4-18. Heat transfer between the
engine and the reservoirs is crossing the boundaries of the system, as the reservoirs are
outside the system.
The Rankine cycle
described in the introductions to Chapters
1 and 2 had four
steps––each with a
well-defined purpose.
Here, we know nothing about the steps––
we know what the
process does, but we
don’t know how.
Heat that is removed
from a system without
being converted into
work or recycled in
any useful way is often
called “spent” or
“rejected” heat.
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178 Fundamentals of Chemical Engineering Thermodynamics
·
QH represents heat
transferred to/from
the “hot” reservoir
·
and QC represents
heat transferred to/
from the “cold”
reservoir.
Step 2 Apply and simplify energy balance
Because this is a steady-state process, the accumulation term in the energy balance is
· =m
· = 0), so the only terms present in
0. The heat engine is also a closed system (m
out
in
·
the energy balance are heat and work. We include two Q terms to represent the heat
exchanged with each of the reservoirs.
·
·
·
(4.110)
0 5 QH 1 QC 1 WS
·
QH is the rate of heat transfer to the engine and is known to be +1000 kJ/min. From our
·
·
sign conventions, QC and WS are both negative (heat transferred out of system, work
done by system on surroundings).
Step 3 Apply and simplify entropy balance
Again, the system is at steady state, so the accumulation term of the entropy
balance is 0. There is no material entering or leaving the system, so the entropy
balance is
·
n5N Q
·
n
05
1 Sgen
(4.111)
T
n51
n
o
·
·
Here there will be two different heat terms, QH and QC.
The fact that the entire process is reversible is significant in this process two different ways:
In a reversible process, no entropy is generated inside the boundaries of the system.
In order for heat transfer to be reversible, the driving forces for heat transfer must
be very small. Consequently, the system temperature at the boundary where the
heat transfer takes places has to be essentially identical to the temperature of the
reservoir itself.
Entropy was first proposed as a property
because Q∙T was
recognized as a useful
quantity in analyzing
heat engines.
Consequently, the entropy balance becomes
·
·
QH QC
05
1
TH
TC
(4.112)
Step 4 Solve entropy balance
By definition, the temperature of a heat reservoir is assumed constant. Since TC , TH , and
·
·
QH are all given, the only unknown in Equation 4.112 is QC, which is the heat transferred
to the cold reservoir.
kJ
·
QC
min
1
800 K
300 K
1000
05
(4.113)
kJ
·
QC 5 2375
min
Step 5 Solve energy balance
The result of step 4 allows us to solve the energy balance for the work.
·
·
·
0 5 QH 1 QC 1 WS
0 5 11000
kJ
kJ
·
2 375
1 WS
min
min
(4.114)
kJ
·
WS 5 2625
min
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C H A P T E R 4 Entropy
Notice that in Example 4-11, we were able to determine the rate at which the heat
engine produced work even though we knew nothing about how, specifically, the
engine functions, other than that it was reversible. Because we assumed nothing
specific, the analysis is valid for any reversible heat engine.
The energy balance for a closed system heat engine (derived in step 2 of
Example 4-11) can be written:
·
·
·
(4.115)
0 5 QH 1 QC 1 Wnet
·
QH represents heat added to the heat engine from a high-temperature reservoir.
·
QC represents heat removed from the heat engine into a low-temperature
reservoir.
·
Wnet represents the total rate at which the engine produces work.
179
This equation was
derived in Example
4-11 for a steady-state
process, but Example
4-12 will show that
substantially the
same equation can be
derived for unsteadystate processes that
are cyclic.
There are typically at least two steps in a heat engine that produce or use work, and
·
the work contributions from these individual steps are combined into the Wnet term.
You might wonder, if all the individual contributions to work are lumped together
·
·
·
·
into Wnet, why aren’t QH and QC combined into a Qnet? One reason is that in the
·
·
entropy balance, QH and QC must be kept distinct, because the temperatures at which
they enter or leave the system are different. Work by contrast does not appear in the
entropy balance at all. Another point is that all heat is not equally useful. Consider
the results from Example 4-11.
·
QH represents 1000 kJ/min of heat available at 800 K, which is valuable and
versatile; it can be used for any heating application in which the desired final
temperature is below 800 K. This heat was likely generated by burning a consumable resource such as fossil fuels.
·
QC represents 375 kJ/min of heat available at 300 K. This heat simply has little or
no practical value. Because 300 K is barely above typical ambient temperatures,
it isn’t realistic to expect there will be many opportunities to transfer this heat
elsewhere in a useful way.
·
·
·
Thus, combining QH and QC into a single “net heat” term (Qnet = 625 kJ/min)
wouldn’t be wrong (at least, it wouldn’t be wrong in the energy balance), but it
would be misleading. We are consuming 1000 kJ/min of a valuable resource—
not 625 kJ/min.
The efficiency of the
heat engine in
Example 4-11 is
62.5%. 1000 kJ/min of
heat is added, of which
62.5% is converted
into work.
PitFaLL
PreventiOn
The purpose of a heat engine is to convert heat energy into work. The efficiency of a
heat engine is to quantify how effectively that is done. Thus,
·
2Wnet
H.E. 5 ·
(4.116)
Qadded
where
η is the symbol conventionally used for efficiency. The subscript H.E. is used to
emphasize that it stands for the efficiency of a heat engine (to distinguish it from,
for example, a turbine or compressor efficiency).
·
Wnet is the net work produced by the heat engine and is typically computed by
summing the individual work contributions of several steps. The negative sign
reflects the fact that W is negative when work is produced by the engine, while
the efficiency is conventionally a positive number.
(Continued)
Equation 4.116 defines
the efficiency of a heat
engine. Equations
4.68 and 4.69 defined
efficiencies for
turbines, pumps and
compressors, which
are often components
of heat engines.
Consequently, in
Chapter 5 we will
often solve problems
that involve more
than one value of
efficiency, and must be
careful to keep them
straight.
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180 Fundamentals of Chemical Engineering Thermodynamics
·
Qadded is the total heat added to the heat engine. In Example 4-11, there was only
one step in which heat was added, and it came from a high-temperature reservoir.
·
·
In this common case, Qadded is simply equal to QH.
·
·
Wnet and Qadded are here expressed as rates (e.g., J/sec), but could also be expressed
as absolute quantities (e.g., kJ), or on the basis of “energy per quantity of operat·
·
ing fluid” (e.g., kJ/kg). Wnet and Qadded must be expressed on the same basis as
each other since efficiency is a dimensionless quantity.
Note that Equation 4.116 does not indicate the form of the work. The following two
examples show that conceptually, heat engines can be used to produce either expansion work (Example 4-12) or shaft work (Example 4-13). In practice, what we need
is more commonly shaft work.
examPle 4‑12
The idea of “isothermal heating” sounds
counter-intuitive, but
if a gas expands as it’s
heated, it transfers the
energy it receives as
heat to the surroundings in the form of
work. Maintaining a
constant temperature
is therefore possible.
the c arnOt cycLe
A gas is confined in a piston-cylinder arrangement. The gas undergoes a series of four reversible
steps, detailed below, which together constitute the Carnot heat engine, or the Carnot cycle.
The steps are illustrated in Figure 4-19 and the state of the gas at each is shown in Figure 4-20.
Isothermal
heating
A
B
TH
QH
Adiabatic
compression
Adiabatic
expansion
TC
TH
TH
D
Section 5.3.3 illustrates that when the
four steps described
here are implemented
in reverse sequence,
they constitute a
Carnot refrigerator.
C
Isothermal TC
cooling
QC
Cold reservior
TC
Image created by John Wetzel, founding author of WikiPremed.
Hot reservior
Figure 4‑19 Schematic of the four steps in the Carnot cycle.
The gas, initially at a high pressure and high temperature (illustrated as point A on
Figure 4-20), is isothermally heated at a constant temperature TH . The gas expands as
it’s heated. The final state is represented by point B on Figure 4-20.
The gas is adiabatically expanded, from state B to state C. In this process, the temperature changes from TH to TC.
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C H A P T E R 4 Entropy
181
A
Isothermal heating
P
Adiabatic
compression
B
Adiabatic expansion
D
C
Isothermal cooling
V
Figure 4‑20 The four steps of a Carnot cycle, using a gas as the
operating fluid, illustrated on a P-V diagram.
The gas is isothermally cooled from state C to state D. In order to maintain a constant temperature, the gas must be compressed while heat is removed.
The gas is adiabatically compressed back to state A.
A. Find a relationship between the net work produced by this process and the measureable properties (P, V, and/or T) of the gas.
We use a P-V plot
rather than (for example) P-T, because this
plot permits a simple
graphical visualization
of work, as explained
in step 2.
B. Find a relationship between the efficiency of the heat engine and the measureable
properties (P, V, and/or T) of the gas.
SOLUTION: No specific conditions are given (e.g., specific values of P or T); we are
looking for mathematical expressions that are true in general for a heat engine of
this kind.
SOLUTION A:
Step 1 Define the system
Define the gas in the cylinder as the system.
Step 2 Assess work
There is no shaft work; only expansion/contraction work. Figure 4-20 demonstrates that
the gas volume changes in all four steps, and therefore work is done either on or by the
system in each step. Recall that expansion/compression work is given by:
WEC 5 2
#
The Carnot cycle is
named after Nicolas
Leonard Sadi Carnot,
a French physicist and
military engineer who
published the first theoretical description of
heat engines in 1824.
(Carnot, 1890)
final state
P dV
initial state
P is the pressure opposing the motion, which in some steps is the gas pressure and in
others is the external pressure. However, in a reversible process, the driving force for any
change is negligible, so external and gas pressures are essentially equal; P can be equated
to the gas pressure for all four steps.
Thus the net work for the complete cycle is:
WEC, net 5 2
# P dV 2 # P dV 2 # P dV 2 # P dV
B
C
D
A
A
B
C
D
(4.117)
Graphically, the work in a particular step is given by the area under the curve representing that step in the P-V diagram, and the total work is the area bounded by the curves
representing the four steps.
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182 Fundamentals of Chemical Engineering Thermodynamics
SOLUTION B:
Step 3 Apply and simplify energy balance
Since there is no specific information on the duration of the process, we start with the
time-independent energy balance. This is a closed system, so energy only enters and
leaves in the forms of work and heat. There is no shaft work; only expansion/contraction
work. Thus, the energy balance is:
U2 2 U1 5 QH 1 QC 1 WEC, net
(4.118)
Step 4 Define the process
This is not a steady-state process; P, V, and T all change with time. However, the process
is cyclic; the gas goes through four steps at the end of which it is returned to its original
state. We could apply the energy balance to any one of the four steps, or to more than
one consecutive step. However, if we define the process as one complete cycle, then the
final state is identical to the initial state. Since enthalpy is a state function, this means
U2 = U1. The energy balance thus further simplifies to
0 5 QH 1 QC 1 WEC, net
(4.119)
2WEC, net 5 sQH 1 QC d
(4.120)
or solving for the net work gives
QC and QH are
opposite in sign, so
QC ∙QH is negative,
making the efficiency
less than one.
This is essentially
identical to the
entropy balance
obtained for the
steady-state process in
Example 4-11. Here it
is only valid if the process is defined as one
complete cycle.
Step 5 Apply definition of efficiency
Dividing both sides of Equation 4.120 by QH and rearranging gives us an expression for
the efficiency of the heat engine:
H.E. 5
2WEC,net
QH
5
QH 1 QC
QH
511
QC
(4.121)
QH
Step 6 Apply and simplify the entropy balance
We start with the time-independent entropy balance. This is a closed system, so min =
mout = 0, and the process is reversible, so Sgen = 0. Thus the entropy balance simplifies to
S2 2 S 1 5
n5N
Qn
n51
n
oT
(4.122)
The entropy balance further simplifies because entropy, like enthalpy, is a state function.
Because the process is one complete cycle, S1 = S2.
Two of the four steps are adiabatic. The other two involve heat transfer, but the system is isothermal during these steps. Thus, the summation becomes
05
QH
TH
1
QC
(4.123)
TC
Solving for QC produces
QC 5
2QHTC
(4.124)
TH
Step 7 Relate efficiency to temperature
Substituting the expression for QC in Equation 4.124 into Equation 4.121 gives us an
expression for the efficiency of the cycle in terms of the temperatures of the heat
reservoirs. We call this the Carnot efficiency
1 T 2
2QHTC
H.E. 5
2WEC, net
QH
511
H
QH
512
TC
TH
(4.125)
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C H A P T E R 4 Entropy
183
Example 4-12 described a series of four reversible steps that are known as the Carnot
cycle and shows expressions for work produced and efficiency when this sequence is
implemented using a gas in a piston-cylinder arrangement. There is no requirement
that the operating fluid be an ideal gas, or even a gas. The next example outlines a
method of implementing a Carnot heat engine that uses water as the operating fluid.
cOntinuOus carnOt cycLe
A Carnot heat engine operates continuously and reversibly at steady state, using the following cycle (illustrated in Figure 4-21):
Saturated liquid water at temperature TH (state A) enters a boiler, and emerges as
saturated vapor (state B). Because the process is a phase change occurring at constant pressure, this is an isothermal heating.
The saturated steam enters a reversible turbine, in which it expands adiabatically to
state C. In this step, some of the steam condenses into liquid.
The saturated liquid/vapor mixture enters a heat exchanger and is isothermally
cooled to state D.
The saturated liquid/vapor mixture enters a reversible pump and an adiabatic compression returns it to state A.
A. What is the physical state of the material (liquid, vapor, mixture of liquid and vapor
etc.) at conditions C and D?
B. What is the efficiency of the heat engine?
400
Temperature (8C)
350
300
TH
A
B
D
C
250
examPle 4‑13
As mentioned in Section 1.2.3, continuous
recirculation of the
operating fluid in a
heat engine (regardless of what that fluid
is) is logical. Any
other design would
involve the continuous expense of adding
fresh material and
disposing of waste
material.
Recall that T H and
T C are defined as the
temperatures of the
hot and cold heat
reservoirs, not of the
system. However,
in a reversible heat
transfer, the reservoir
and system temperatures are essentially
identical.
200
150
100
TC
50
0
0
2
4
6
8
10
Specific entropy (kJ/kg-K)
Figure 4‑21 The four steps in a Carnot cycle, using H2O as the
operating fluid, illustrated on a T-Ŝ diagram.
SOLUTION A:
Step 1 Apply entropy balance to single steps
Figure 4-21 shows the process sketched on a temperature-entropy diagram. States A and
B are saturated liquid and saturated vapor, respectively, both at temperature TH. Because
the transition between state C and state D is an isothermal cooling, we know that states
C and D must both be at the same temperature as each other, specifically TC.
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184 Fundamentals of Chemical Engineering Thermodynamics
The transition between states B and C is carried out in an adiabatic, reversible,
steady-state process. However, it is an open system; saturated steam continuously enters and continuously leaves as a liquid–vapor mixture. Consequently, as in the nozzle in
Example 4-6, the entropy balance simplifies to
· Ŝ 2 m
· Ŝ
05m
(4.126)
in
in
out
out
Since the mass flow rates entering and leaving must be the same at steady state, equation
4.126 demonstrates that the specific entropy must be equal for the entering and exiting
material. Thus, the transition from state B to C must be represented by a vertical line on
Figure 4-21. Identical reasoning applies to the transition from state D to A. Examining
Figure 4-21, states C and D are both liquid–vapor mixtures with the same specific entropies as states B and A, respectively.
SOLUTION B:
Examples 4-11
through 4-13 consider
three different reversible heat engines. This
entropy balance has
been valid for all of
them, but only when
specific definitions
of “the system” were
employed.
Step 2 Apply entropy balance to entire process
In step 1 we established that a single component of the heat engine is an open
system. However, the heat engine as a whole is a closed system; the H2O continuously
recirculates inside the boundaries of the system but no material enters or leaves. The
process is also steady state (accumulation = 0) and reversible (Sgen = 0). Consequently,
for the whole process, the entropy balance is
·
·
QH QC
05
1
(4.127)
TH
TC
Step 3 Apply energy balance to entire process
For a steady-state process in a closed system, the energy balance consists of only work
and heat:
·
·
·
(4.128)
0 5 QH 1 QC 1 WS,net
The energy and entropy balances are identical to the ones in Example 4-12, aside from
the fact that the process in Example 4-12 produced expansion work while this one produces shaft work. Consequently, the subsequent derivation of the efficiency of the heat
engine is also identical.
·
2WS, net
TC
5
512
·
TH
QH
We note several differences between the processes in Examples 4-12 and 4-13.
In Example 4-12, the four steps are applied sequentially to a gas confined
in a chamber. In Example 4-13, the operating fluid continuously recirculates
through the four processes, which are carried out continuously at steady
state.
In the piston-cylinder process, the operating fluid is always a gas, and the circulating system uses liquid water and water vapor at different points.
The piston-cylinder maintains the constant temperature in the isothermal steps
by adding or removing work, thus balancing the heat that is removed or added.
In the circulating system, the heat energy added or removed produces a phase
change, which is an isothermal process. There is no need to add or remove work.
The piston-cylinder process yields expansion work; the circulating process yields
shaft work.
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C H A P T E R 4 Entropy
Despite these differences, the four steps in both cycles mirror each other. Both processes are composed of an isothermal heating, adiabatic expansion, isothermal cooling, and adiabatic compression. Any reversible cycle comprised of these four steps is
a Carnot heat engine.
The efficiency expression derived in Example 4-12 was also valid for Example
4-13 and in fact applies to any Carnot engine.
Carnot 5 1 2
TC
TH
5
TH 2 TC
TH
(4.129)
The efficiency of a
Carnot heat engine
depends only upon the
temperatures of the
heat reservoirs, and
not upon the working
fluid or the details of
how the process is
carried out.
This Carnot efficiency represents the highest possible efficiency for a heat engine
operating between the the temperatures T H and T C. The Carnot cycle is a fully
reversible process, and is thus an idealized limit which cannot be fully attained in
practice. Section 5-2 explores the design of real heat engines, and how they can most
closely approximate the Carnot cycle.
4.6 s u M M a r y O F c h a P t e r F O u r
A reversible process is one in which all driving forces for changes are negligible.
Entropy is a state property that can be regarded as a metric for lost work. It is
mathematically defined as
dS 5
185
dQrev
T
Specific entropy has dimensions of energy per unit mass per temperature (e.g.,
kJ/kg · K).
The entropy of the universe in unchanged by any reversible process, and is
increased by any irreversible process.
Irreversible processes lead to lost work, and entropy can be regarded as a property that quantifies the loss of potential to do work.
A macrostate is a specific description of the state of a system.
A microstate is a specific configuration for the components that make up the
system.
Entropy of a system can be computed from the number of distinct microstates
that each correspond to the macrostate that describes the system, through
Equation 4.36.
The entropy balance takes the form Accumulation = In – Out + Generation.
The generalized entropy balance (Equations 4.37, 4.38, or 4.39) can be applied to
any system, and can be used to establish limitations on what is possible within a
process.
Accumulation of entropy is zero in any steady-state process.
Generation of entropy is zero in any reversible process.
Because entropy is a state property, the change in entropy of a system can be
determined by calculating the change in entropy along a hypothetical reversible
path from the initial state to the final state. This is valid whether the real path is
reversible or not.
The efficiency of a real process can be defined by comparing it to a hypothetical
reversible process.
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186 Fundamentals of Chemical Engineering Thermodynamics
4.7 e x e r c i s e s
4-1.
4-2.
Several processes are described. Indicate whether
each is reasonably modeled as reversible, and if
not, indicate what aspect of the process makes it
irreversible.
A. The inside of a refrigerator is at exactly 31.9°F. A
glass of room temperature water is placed inside
the refrigerator and left there until it is frozen.
B. The inside of a refrigerator is at exactly 31.9°F.
A glass containing an equilibrium mixture of ice
and water is placed inside the refrigerator and
left there until it is frozen.
C. The inside of a refrigerator is at exactly 32.1°F. A
glass containing an ice–water mixture is placed
inside the refrigerator and left there until the ice
is all melted.
D. A diver is 1000 feet below the surface of the
ocean, where the pressure is 31 atm. She rapidly ascends to the surface, where the pressure is
1 atm.
E. A diver is 1000 feet below the surface of the
ocean, where the pressure is 31 atm. Over a period of half an hour she gradually ascends to the
surface, where the pressure is 1 atm.
Find the change in entropy for the system in each of
the following processes.
A. The system is 5 kg of initially saturated steam at
P = 1 bar, and it is heated isobarically to 300°C.
B. The system is 5 kg of steam, initially at P = 5 bar
and T = 300°C, and it is cooled and condensed
into saturated liquid at P = 5 bar.
C. The system is 10 moles of an ideal gas with
CV* = 1.5 R, and it is compressed from T = 400 K
and P = 1 bar to T = 500 K and P = 3 bar.
D. The system is 10 lb-mol of an ideal gas with CP*
= 3.5 R, and it is expanded from T = 500°F and
P = 4 atm to T = 100°F and P = 1 atm.
4-3.
A heat reservoir is at 50°C. Find the change in
entropy of the heat reservoir when
A. 1 kJ of heat is added.
B. 1 kJ of heat is removed.
C. Heat is added at a rate of 100 W for a total of
one hour.
4-4.
The system is 100 grams of a solid that has
ĈP = 0.3 J/g · K. Find the change in entropy when
A. It is heated reversibly from 300 K to 500 K.
B. It is cooled reversibly from 50°C to 25°C.
C. It is heated reversibly from 50°F to 100°F.
D. It is cooled reversibly from 50°F to 0°F.
4-5.
The system is a 5 lbm object that has ĈP = 0.1 BTU/
lbm · °R. Find the change in entropy when:
A. It is heated reversibly from 500°R to 1000°R.
B. It is cooled reversibly from 200°F to 50°F.
C. It is heated reversibly from –5°F to 100°F.
D. It is cooled reversibly from 50°C to 0°C.
4-6.
The system is 1 kg of nitrogen, initially at T 5 300 K
and P 5 4 bar. For each of the following cases,
find the change in entropy of the system using
Figure 2-3, then find the change in entropy again by
assuming nitrogen is an ideal gas and using the data
in Appendix D.
A. The nitrogen is isobarically heated to 330 K.
B. The nitrogen is isobarically cooled to 225 K.
C. The nitrogen is isothermally compressed to P =
10 bar.
D. The nitrogen is compressed to T 5 330 K and
P 5 10 bar.
4-7.
Find the efficiency of each of the following Carnot
heat engines.
A. TH = 500 K, TC = 300 K
B. TH = 500°C, TC = 300°C
C. TH = 1000°F, TC = 500°F
D. TH = 1000°R, TC = 500°R
4-8.
A system is operating at a steady state. Find the rate
of at which entropy is generated inside the system
for each of the following cases.
A. The system is adiabatic. 100 kg/min of saturated
steam enters at 5 bar and 100 kg/min of saturated
steam leaves at 1 bar.
B. The system is adiabatic. 100 kg/min of saturated
liquid water enters at 1 bar and 100 kg/min of
liquid water leaves at T = 120°C and P = 10 bar.
C. The system is closed. 1000 kJ/s enter the system
at a boundary where T = 1000 K, and 750 kJ/s
exit the system at a boundary where T = 500 K.
D. The system is closed. 1000 kJ/s enter the system at
a boundary where T = 1000 K, and 500 kJ/s exit
the system at a boundary where T = 750 K.
4.8 P r O b L e M s
4-9.
Steam enters a turbine at 10 bar. The effluent pressure is 1 bar and the efficiency of the turbine is
80%. Determine the state of the turbine effluent
(if pure liquid or vapor, find the temperature, and
if a mixture, find the quality) and compute the work
produced per kg of entering steam, when
A. The entering steam has T = 250°C.
B. The entering steam has T = 325°C.
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C H A P T E R 4 Entropy
4-10. A series of two turbines and a heat exchanger are
used to obtain shaft work from steam in a steadystate process. The steam enters the first turbine at
10 bar and 250°C, and exits the turbine at 3 bar. In
the heat exchanger, the steam is heated back up to
250°C, while the pressure remains 3 bar. This steam
enters the second turbine in which it is expanded
to 1 bar. Each turbine has an efficiency of 80%. For
each kilogram of steam entering the process, find the
amount of work produced in each turbine, and the
amount of heat added in the heat exchanger.
4-11. Compare the results of Problems 4-9 and 4-10.
A. Without the heat exchanger, the pair of turbines
in Problem 4-10 would function identically to
the turbine in Problem 4-9A. Comment on the
effects the heat exchanger has on the process.
B. If the heat exchanger in Problem 4-10 was
moved in front of the first turbine, and the two
turbines were combined, the result would essentially be the process examined in Problem 4-9B.
The process in Problem 4-9B looks simpler: the
same heat is added, it’s just being added sooner,
so any benefits you noted in part A of this problem are still obtained, and there’s no need for
two separate turbines. Can you see any rationale
for using the apparently more complicated process described in Problem 4-10?
4-12. A 10 ounce glass of water (half full) is initially at
15°C. It is left outside overnight in a location where
the air temperature is –5°C. By morning the glass
is in equilibrium with the surroundings. Find the
change in entropy of the water, the surroundings, and
the universe for this process. Assume for liquid water
ĈP ~ ĈV ~ 4.19 J/g · K and for ice ĈP ~ ĈV ~ 2.11 J/g · K.
4-13. An inventor claims to have built a machine that
operates as follows:
100 kg/min of steam at T = 400°C and P = 5 bar
enters the machine.
100 kg/min of saturated liquid water at 40°C
leaves the machine.
50,000 kJ/min of shaft work is produced by
the system. There is no heat exchange with
surroundings.
An undisclosed amount of nitrogen enters the
process at 315 K and 1 bar, and the same amount
of nitrogen leaves at 330 K and 1 bar.
Kinetic and potential energy of all entering and
leaving streams is negligible.
This is a steady-state process that can continue as
described above indefinitely.
Without knowing anything about how the machine
works, what can you say about the feasibility of the
inventor’s claim?
187
4-14. Steam at T = 400°C and P = 5 bar enters an adiabatic, steady-state nozzle. The exiting pressure is
P = 1 bar.
A. If the nozzle is reversible, find the temperature
and velocity of the exiting steam.
B. If the exiting temperature is T = 350°C, then the
nozzle is identical to the one that was examined
in Example 3-4. Find the amount of entropy generated in this nozzle for each kilogram of entering steam.
4-15. Prove that the process shown in Figure 4-9 is impossible if the cylinder contains a monatomic ideal gas.
4-16. Ten moles of a gas are placed in a rigid container.
Initially the gas is at P = 0.5 bar and T = 300 K and
the container is also at T = 300 K. The container
is placed in a furnace, where its surroundings are at
a constant T = 600 K. The container is left in the
furnace until both the container and the gas inside
it reach thermal equilibrium with the surroundings.
The gas can be modeled as an ideal gas at the pressures attained throughout this process, and has a
constant heat capacity of CV* = 2.5R. The container
itself has a mass of 10 kg (not including the mass of
the gas inside) and a heat capacity of ĈV = 1.5 J/g · K.
A. Find the heat added to the gas.
B. Find the heat added to the container.
C. Find the change in entropy of the gas.
D. Find the change in entropy of the universe.
E. What aspect of this process is irreversible?
4-17. This problem re-examines the dirigible in Example 2-4. The balloon contains 5000 moles of helium,
which can be modeled as an ideal gas with CP* 5
(5/2)R at the conditions in this problem. The dirigible is initially at T 5 25°C and P 5 1 atm. The
dirigible flies slowly enough that all changes to the
helium are reversible. Find Q, W, ΔU, ΔH, and ΔS for
the helium if its final state is:
A. T 5 15°C and P 5 1 atm. Pressure is uniform
throughout the process.
B. T 5 25°C and P 5 0.9 atm. Temperature is uniform throughout the process.
4-18. Problem 3-19 described four different adiabatic mixing processes, and the relevant data presented was
that the enthalpy of fusion of water is 333.55 J/g, the
heat capacity of ice is constant at 2.11 J/g · K, and the
heat capacity of liquid water is constant at 4.19 J/g · K.
Find the change in entropy of the universe resulting from each of these processes, and comment on
whether each result is consistent with whether the
process appears to be reversible or irreversible.
A. 100 g of liquid water at 50°C is mixed with 50 g of
liquid water at 0°C.
B. 100 g of liquid water at 50°C is mixed with 50 g of
ice at 0°C.
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188 Fundamentals of Chemical Engineering Thermodynamics
C. 50 g of liquid water at 0°C is mixed with 50 g of
ice at 0°C.
D. 50 g of liquid water at 20°C is mixed with 50 g of
ice at –20°C.
4-19. A 10 kg copper block has an initial temperature of
800 K. It is placed in a well-insulated vessel containing 100 kg of water initially at 290 K. The process is isobaric at atmospheric pressure. Assume
copper and liquid water have constant heat capacity of CP = 4.184 J/g · K for water and 0.398 J/g · K
for copper.
A. When the system reaches equilibrium will be the
temperature of the block?
B. What is the change in entropy of the universe
resulting from the process?
4-20. A stream of liquid nitrogen enters an adiabatic,
steady-state valve as a saturated liquid at P = 2 MPa.
The material leaves the valve at P = 0.6 MPa. Use
the data in Figure 2-3 to determine the following.
A. The temperature of the nitrogen leaving the
valve.
B. The physical state of the nitrogen leaving the
valve (if VLE mixture, indicate quality).
C. The rate at which entropy is generated in the
valve (per kg of entering nitrogen.)
D. The irreversible feature of the process, if any, that
explains the result of part C.
E. Repeat parts A, B, and C for methane instead of
nitrogen. Use Figure 7-1.
4-21. One of the steps in a typical refrigeration process
is a boiler. Freon® 22 enters a steady-state boiler
as a VLE mixture with a pressure P = 0.5 bar and
quality of q = 0.2, and leaves the boiler as saturated
vapor at P = 0.5 bar. Use the data in Appendix F to
determine the following.
A. How much heat is added to the boiler per kilogram of entering Freon® 22.
B. The flow rate of Freon through the boiler if the
·
rate of heat addition is Q 5 200 kJymin.
4-22. One of the steps in a typical refrigeration process is
a compressor. Freon® 22 enters a steady-state compressor as saturated vapor at P = 0.5 bar, and leaves
at P = 3 bar. Use the data in Appendix F for the
following.
A. Determine the work added to the compressor,
per kilogram of entering Freon, if the compressor is reversible.
B. Determine the work added to the compressor,
and the temperature of the exiting Freon, if the
compressor has an efficiency of 75%.
4-23. One of the steps in a typical refrigeration process
is a condenser. Freon® 22 enters a condenser at
steady state as superheated vapor at P = 3 bar and
T = 30°C. It leaves the condenser as saturated liquid
at P = 3 bar. Use the data in Appendix F for the
following.
A. Find the heat removed from the condenser, per
kilogram of entering Freon.
B. Find the flow rate of Freon in the condenser, if
the rate at which heat is expelled from the con·
denser is Q 5 250 kJ/min.
C. Determine the change in entropy of the universe
resulting from the process in part B, assuming the
coolant used for the condenser is a heat reservoir
at a temperature 5°C lower than the temperature
of the Freon exiting the condenser.
4-24. Four eggs labeled A, B, C, and D are to be placed in
three bowls labeled 1, 2, and 3.
A. How many distinct “microstates“ are there?
B. How many distinct “macrostates” are there? List
the macrostates, and indicate how many microstates there are which produce each macrostate.
4-25. You have four molecules labeled 1, 2, 3, and 4 and
two bins labeled A and B. Any number of molecules
can be placed in each bin.
A. List all of the possible microstates. How many
are there?
B. List all of the macrostates, and indicate which
microstates correspond to each if the four molecules are all the same compound.
C. List all of the macrostates, and indicate which
microstates correspond to each, if molecules 1, 2,
and 3 are the same as each other but 4 is a different compound.
4-26. A high-temperature reservoir at T 5 375°C is to
be used as the heat source for a steady-state heat
engine, and a low-temperature reservoir at T 5 25°C
is to be used as the heat sink. The boiler takes in
liquid water at P 5 8 bar and T 5 42°C and expels
steam at P 5 8 bar and T 5 350°C. The condenser
takes in steam at T 5 100°C and P 5 0.1 bar and
expels saturated liquid water at P 5 0.1 bar. The
mass flow rate of the steam throughout the process
is 5 kg/s. Find the rate at which the entropy of the
universe is increased by the processes in the boiler
and the condenser.
4-27. A high-temperature reservoir at T 5 500°F is to
be used as the heat source for a steady-state heat
engine, and a low-temperature reservoir at
T = 75°F is to be used as the heat sink. The boiler
takes in a liquid organic compound at P 5 5 atm and
T 5 100°F and expels the organic compound as a
vapor at P 5 5 atm and T 5 470°F. The condenser
takes in the organic compound as a saturated vapor
at T 5 100°F and expels it as a saturated at T 5 100°F.
The mass flow rate of the organic compound throughout the process is 500 lbm/min. Relevant data for the
organic compound include:
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C H A P T E R 4 Entropy
Boiling point at P = 5 atm is 300°F.
DĤf = 500 BTU/lbm at T = 100°F and DĤf =
450 BTU/lbm at T = 300°F.
Liquid phase ĈP = 1.2 BTU/lbm.
Vapor phase heat capacity at P = 5 bar is ĈP =
1.5 + 0.003T with T expressed in Rankine and
CP expressed in BTU/lbm · °R.
A. Find the efficiency of a Carnot heat engine operating between 500°F and 75°F.
B. Find the efficiency of a Carnot heat engine operating between T = 470°F and T = 100°F.
C. Find the rate at which the entropy of the universe
is increased by the processes in the boiler and the
condenser.
4-28. Two chambers are separated by a partition. One
of the chambers is evacuated, and the other has a
volume of 1 m3 and contains steam at T 5 500°C
and P 5 10 bar. The partition is removed, allowing
the gas to expand into the evacuated chamber. No
heat transfer occurs with the surroundings. The final
steam pressure is P = 4 bar. Find
A. The mass of steam.
B. The volume of the chamber that was initially
evacuated.
C. The change in entropy of the universe resulting
from the process.
D. Re-do parts A–C, but this time, instead of steam,
assume the 1 m3 chamber was filled with an ideal
gas with Cp* = (7/2)R.
4-29. Throughout Chapters 3 and 4, we have assumed
that potential and kinetic energy are negligible in
standard chemical process equipment. This problem tests how valid these approximations are for
a turbine. Steam at 14 bar and 300°C enters a turbine through a 5 cm diameter pipe at a velocity of
2.5 m/s. The exhaust exits via a 20 cm diameter
pipe that is located 1.5 m below the inlet pipe. The
exhaust is saturated steam at 0.9 bar. Assume steadystate, adiabatic operation, but assume nothing else
about various contributions to the energy balance.
A. Determine the mass flow rate entering the turbine.
B. Determine the power output from the turbine.
C. Determine the efficiency of the turbine.
4-30. Saturated liquid water enters a steady-state, adiabatic throttling valve at T = 100°C . The pressure of
the fluid leaving the valve is P = 0.5 bar. It has been
suggested that perhaps it would make more sense to
replace the valve with a turbine––the desired pressure drop will still occur, but some work would be
obtained from the turbine.
A. Determine the physical state of the fluid leaving
the valve.
B. Determine the rate at which entropy is generated
in the valve per kilogram of entering water.
189
C. Determine the maximum work that could be
produced by a turbine per kilogram of entering water if the stream entering the turbine was
identical to the stream entering the valve, and the
pressure leaving the turbine was P = 0.5 bar.
D. Compare the physical state of the fluid leaving
the “idealized” turbine described in part C to
the physical state of the fluid leaving the valve as
described in part A. Are they identical? Can you
rationalize why they are, or are not, identical?
4-31. Superheated steam enters a steady-state, adiabatic
throttling valve at T = 200°C and P = 1 bar. The
pressure of the fluid leaving the valve is P = 0.5 bar.
It has been suggested that perhaps it would make
more sense to replace the valve with a turbine—the
desired pressure drop will still occur, but some work
would be obtained from the turbine.
A. Determine the physical state of the fluid leaving
the valve.
B. Determine the rate at which entropy is generated
in the valve, per kilogram of entering steam.
C. Determine the maximum work that could be
produced by a turbine, per kilogram of entering steam, if the steam entering the turbine was
identical to the steam entering the valve, and the
pressure leaving the turbine was P = 0.5 bar.
D. Compare the physical state of the fluid leaving
the “idealized” turbine described in part C to
the physical state of the fluid leaving the valve as
described in part A. Are they identical? Can you
rationalize why they are or are not identical?
E. Compare your answers to this problem to your
answers to Problem 4-30. How is the outcome
for steam different from the outcome of a similar
process involving liquid water?
4-32. The thermostat in a house is set at 70°F. Consequently, the inside of the house is always at 70°F,
regardless of the season.
A. One day, the outside temperature is 50°F, and the
house loses 100 kJ of heat to the surroundings.
What is the change of entropy for the universe
resulting from this process?
B. Besides the heat transfer noted in part A, is there
any other source of entropy generation associated with maintaining this house at 70°F?
C. If a Carnot engine were designed to operate with
the house as the heat source and the outdoors as
the heat sink, what would its efficiency be on this
day? How much work would be produced if the
100 kJ was sent to this Carnot heat engine, rather
than simply being lost to the outdoors?
4-33. A nuclear power plant generates 750 MW of power.
The heat engine uses a nuclear reactor operating
at 315°C as the source of heat. A river is available
(at ~20°C) which has a volumetric flow rate of
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
190 Fundamentals of Chemical Engineering Thermodynamics
Initially, the temperature increases steadily while
the volume increases very little. At a temperature of
T = 97°F, the volume of the cylinder is 89 cm3. At
T = 97.2°F, the temperature levels off, and the volume begins to expand more rapidly. 89 kJ of heat are
added between the time the temperature levels off
and the time the temperature begins to increase again.
Unfortunately, you don’t have an accurate recording of the volume at the exact time the temperature
started increasing. An additional 1.3 kJ of heat are
added during the time the temperature climbs from
97.2°F to 100°F, at which time you stop the experiment.
Give your best estimates of ΔHvap, ΔUvap, and ΔSvap
and CP*.
165 m3/s. If you use the river as a heat sink, estimate
the temperature rise in the river at the point where
the heat is dumped. Assume the actual efficiency
of the plant is 60% of the Carnot efficiency.
4-34. You are designing a chemical process that involves
a relatively unstudied compound, and you need to
know Δ Hvap, ΔUvap and ΔSvap, and CP* for the compound at atmospheric pressure. You place 1 mole
of the compound (in the liquid state) in a pistoncylinder device and heat it very slowly while maintaining the pressure at 1 atm. You can’t actually see
what’s going on inside the cylinder, but you can
monitor the heat addition, temperature, and volume.
4.9 g LO s s a r y O F s yM b O L s
CP
constant pressure heat capacity (mole basis)
n·
molar flow rate
P
pressure
C *P
constant pressure heat capacity (mole basis) for ideal gas
Q
CV
constant volume heat capacity (mole basis)
QC
C *V
constant volume heat capacity (mole basis) for ideal gas
QH
ĈV
constant-volume heat capacity (mass basis)
·
Q
·
QC
Ĉp
constant-pressure heat capacity (mass basis)
·
QH
H
enthalpy
Ĥ
specific enthalpy
H
molar enthalpy
M
mass of system
m
mass added to or removed
from system
·
m
R
TH
temperature of a hightemperature heat reservoir
heat
t
time
heat exchanged with a lowtemperature heat reservoir
U
internal energy
Û
specific internal energy
heat exchanged with a hightemperature heat reservoir
U
molar internal energy
V
volume
rate of heat addition
V̂
specific volume
rate of heat exchange with
low temperature reservoir
V
molar volume
v
velocity
W
·
W
work
WEC
work of expansion/contraction
rate of heat exchange with
low temperature reservoir
gas constant
power
S
entropy
Ŝ
specific entropy
WS
shaft work
efficiency
S
·
Sgen
molar entropy
rate of entropy generation
H.E.
mass flow rate
T
temperature
overall efficiency of a heat
engine
number of moles in system
overall efficiency of a completely reversible heat engine
n
number of moles added to or
removed from system
temperature of a lowtemperature heat reservoir
carnot
N
TC
4.10 r e F e r e n c e s
Carnot, S.; Thurston R. (editor and translator), Reflections
on the Motive Power of Heat and on Machines Fitted to
Develop That Power, John Wiley & Sons, 1890.
Gutman, P.; Djurdjevic, I., “A simple method for showing entropy is a function of state,” Journal of Chemical
Education, 65, 1988.
Sandler, S. An Introduction to Applied Statistical Thermodynamics, John Wiley & Sons, 2011.
Vemulapalli, G. K., “A Simple Method for Showing
Entropy is a Function of State,” Journal of Chemical
Education, 63, 1986.
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Thermodynamic Processes
and Cycles
5
Learning Objectives
This chapter is intended to help you learn to:
Apply entropy and energy balances to larger chemical processes with multiple unit
operations
Solve energy and entropy balances to fully characterize such processes
Recognize the purposes of Rankine heat engines, refrigerators, and Linde liquefaction
processes and appreciate the role of each unit operation in these processes
Propose preliminary designs of Rankine heat engines, refrigerators, and Linde
liquefaction processes that meet given specifications and operate within given
constraints
Compare and evaluate process designs, using appropriate benchmarks such as
efficiency for heat engines and coefficient of performance for refrigerators
C
hapters 3 and 4 examined the application of material, energy, and entropy balances
to a variety of systems. Most of the examples focused on comparatively simple systems, such as single pieces of equipment. This chapter examines combining individual unit operations into complete processes, and analyzing these processes using material,
energy, and entropy balances.
5.1 MOtivatiOnaL exaMPLe: chemical Process design
Chemical engineers can and do work in a great variety of industries and in a great
variety of roles. However, one cross-cutting aspect of the practice of chemical
engineering is the design and analysis of complete chemical processes. As an
example, we examine the problem of synthesizing ammonia. Imagine you were tasked
with designing a continuously operating, steady-state chemical process that makes
30 million lbm/year of ammonia. Imagine further that all you really knew about
ammonia synthesis to begin with was the information given in Problem 2-16, where
nitrogen and hydrogen can be combined at high pressure over an iron catalyst. Your
first step might be to draw a basic block diagram like that shown in Figure 5-1.
191
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192 Fundamentals of Chemical Engineering Thermodynamics
Recycled N2,H2
Figure 5-1 describes
ammonia synthesis
and is quite similar
to Figure 1-1, which
described chemical
processes in general.
Nitrogen
Hydrogen
Reactor
NH3
Separations
N2 1 3H2 5 2NH3 Unreacted N2,H2
Product NH3
Figure 5‑1 Preliminary schematic of process for ammonia
synthesis.
Ultimately, we need to identify and design each of the individual unit operations
in the process, but right now we have nothing more than boxes labeled “Reactor” and
“Separations.” Developing a complete design will require a great many engineering
decisions. Let’s start at the beginning: where does the nitrogen come from? A natural
answer is “air contains almost 80% nitrogen and is free.” This simple answer affects
our vision of the process in at least two ways.
1. If we use air as a feedstock, then oxygen is entering the process, along with other
impurities.
2. Air enters the process at ambient T and P, but the reaction is carried out at “high”
pressure.
Thus, we could revise our block diagram into the one shown in Figure 5-2, in which
the nitrogen is purified and compressed before entering the reactor. Figure 5-3
shows another alternative: rather than designing a separation process to purify the
nitrogen, send compressed air directly to the reactor, and allow the oxygen to circulate through the process with the nitrogen. The alternative in Figure 5-3 looks like
a simpler process requiring less equipment, so offhand we might expect it’s cheaper
to build. But which would be cheaper to operate on a day-to-day basis? On the one
hand, shaft work is expensive, and the process in Figure 5-3 involves compressing
not only the nitrogen and hydrogen, but also oxygen. On the other hand, the “purification” step in Figure 5-2 might require work of its own. Finally, we don’t yet know
whether the presence of oxygen would have any negative effects on the operation
of the reactor.
Recycled N2,H2
Reactor
H2
Air
NH3
N2 1 3H2 5 2NH3 Unreacted N2,H2
Purification
Separations
Compression
Product NH3
O2,Ar, any other impurities
Figure 5‑2 Schematic of the ammonia synthesis process in which nitrogen is
purified before being fed to the reactor.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 5 Thermodynamic Processes and Cycles
193
Purge
Recycled N2,H2,O2,Ar
Air
Reactor
Compression
H2
NH3
N2 1 3H2 5 2NH3 Unreacted N2,H2
Separations
O2,Ar
Product NH3
Figure 5‑3 Schematic of the ammonia synthesis process in which
compressed air is fed, without separation, to the reactor.
The foregoing discussion illustrates the significant challenge of designing a complete process. In considering the question “where does the nitrogen come from,” we
have raised several new questions, and are already wrestling with two fundamentally
different approaches, between which we must somehow choose. The next question
could be “Where does the hydrogen come from?” but we will not explore that at this
time. Our goal with this discussion is not to reach any final conclusions about ammonia synthesis but to consider What is the role of thermodynamics in the development
of complete processes? Before reading further, try putting some thought into how
what we’ve learned so far applies to the design of this ammonia synthesis process.
Up to this point, we have primarily applied the principles of material, energy,
and entropy balances developed in Chapters 3 and 4 to simple systems, such as single
pieces of equipment. However, we can apply these same balance equations to any
system—including large systems like an entire chemical plant or the entire world.
Notice the system boundaries drawn in Figure 5-2. Even without making any decisions about how the process actually works, we can make significant determinations
by applying material balances to this system.
It takes 24.7 million lbm/year of nitrogen and 5.3 million lbm/year of hydrogen to
make 30 million lbm/year of ammonia.
Modeling air as 78 mol% nitrogen, 21 mol% oxygen, and 1 mol% argon, we can
show that the “air entering” stream must contain 7.6 million lbm/year of oxygen
and ~200,000 lbm/year of argon, along with 24.7 million lbm/year of nitrogen.
Writing and solving material balances for the system shown in Figure 5-3 is much
more complicated due to the presence of the purge stream, but another perspective
is that material balances are why we know the purge stream is necessary to begin
with. If the purge stream were not present, writing an oxygen balance for this system would reveal that oxygen enters the system but has no way to leave. Achieving
a steady state would therefore be impossible; oxygen and the other inerts would
accumulate in the system, producing unsafe pressure increases.
In summary, useful insights can be gained from applying balance equations to
complete processes. Looking into the design of individual steps in the process, the
principles of thermodynamics are again evident.
In Example 4-11, we
applied energy and
entropy balances to a
heat engine, without
knowing how it actually worked. Similarly,
here we apply material
balances to an opensystem chemical
synthesis process,
even though we know
nothing yet about the
inner workings of the
process.
FOOd FOr
thOught
5-1
Do for yourself
the calculations that
produce these
numbers.
Some combination of compressors and heat exchangers will presumably be used
to deliver the feedstock gases at the desired temperature and pressure to enter
the reactor. We have examined compressors and heat exchangers individually in
Chapters 3 and 4. In this chapter Example 5-7 studies the design of a series of
heat exchangers and compressors.
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194 Fundamentals of Chemical Engineering Thermodynamics
Ammonia has a much higher boiling point than oxygen, nitrogen, or hydrogen
(some of the relevant data was shown in Table 2-1). Ammonia is also highly soluble in water, while oxygen, nitrogen, and hydrogen gases are minimally soluble
in water. Either of these facts can logically be considered as the basis for designing the “separation” step. In either case, the principles of multi-component
phase equilibrium covered in Chapters 10 through 12 would be crucial for understanding and designing the process.
Chapters 1 through 4 present a basic “toolbox” of thermodynamic principles. The
foregoing discussion outlines how these “tools,” and additional tools we will develop
in subsequent chapters, can be applied to exactly the types of problems chemical engineers are tasked with solving. In this chapter, we will begin to examine complete, integrated processes that are comprised of multiple unit operations. We are not yet ready
to further our analysis of the ammonia synthesis process specifically—we have only
covered modeling of pure compounds, and the reaction and separation processes
involve mixtures. We will re-visit the ammonia synthesis challenge in Chapter 15,
at which point we will have covered both chemical reaction equilibrium and multicomponent equilibrium. For now, we begin our study of process analysis and design
by revisiting the Rankine heat engine, which was introduced in Sections 1.2.3 and 2.1.
5.2 real heat engines
The Carnot cycle is
analogous to the “hypothetical reversible
paths” in Examples
4-4 and 4-5, in that
while it exists only in
our imagination, we
can learn things from
it that are useful in
solving real problems.
The Rankine cycle was
discussed in Chapters
1 and 2 because it
provided a good
framework for introducing a number of
thermodynamic concepts. At this point, we
are ready to analyze a
Rankine heat engine
in a more thorough
and quantitative way.
Section 4-5 introduced the Carnot cycle—a completely reversible heat engine. Unfortunately, realistically speaking, it is not possible to build a reversible heat engine.
For example, if the addition of heat were truly reversible, this would mean the driving force for heat transfer is infinitesimally small, which in turn means the heat transfer is infinitesimally slow. The Carnot heat engine is thus a hypothetical, idealized
case that can’t be implemented in the real world.
However, the Carnot cycle is a useful theoretical construct because it represents
the maximum conversion of heat into work that is possible for a heat engine operating between hot and cold reservoirs of two particular temperatures. Thus, it makes
sense to design real heat engines so that they approximate Carnot heat engines as
closely as is realistically possible.
Consequently, Section 5.2.1 examines real heat engines that are designed using
the Rankine cycle and how design decisions can be understood through consideration of, and comparison to, the Carnot cycle.
5.2.1 comparing and contrasting the rankine cycle
with the carnot cycle
The Rankine cycle was introduced in Section 1.2. It operates as given here.
High-pressure liquid enters a boiler, absorbs energy from a heat source, and
emerges as either saturated vapor or superheated vapor.
The high-pressure vapor enters a turbine, producing work and emerging as lowpressure vapor, possibly with a small amount of liquid entrained in the vapor.
The low-pressure vapor enters a condenser, emits energy to a heat sink, and
emerges as saturated liquid.
The low-pressure liquid enters a pump, is compressed into high-pressure liquid,
and is returned to the boiler.
The entire process is typically designed to operate at steady state.
Water is typically used as the operating fluid, though the logic of the four steps
is equally valid for other operating fluids.
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C H A P T E R 5 Thermodynamic Processes and Cycles
195
Figure 5-4 shows the steps of the Carnot cycle, and Figure 5-5 the steps of the
Rankine cycle, each sketched on a T vs. Ŝ diagram for water. Here, we will review
the Carnot cycle as it was described in Example 4-13, and consider what aspects of the
Carnot cycle are, and are not, realistic to implement in real heat engines.
400
350
Temperature (8C)
300
TH
4
1
250
200
150
100
50
0
Tc
3
0
2
2
6
4
8
Tc
10
Specific entropy (kJ/kg-K)
Figure 5‑4 An example Carnot heat engine cycle, illustrated on the T-Ŝ plot for
water. State 1 is the steam after isothermal heating, 2 is the liquid/vapor mixture
after adiabatic expansion, 3 is the liquid/vapor mixture after isothermal cooling, and
4 is the liquid after adiabatic compression. Recall that the entire cycle is reversible;
consequently the adiabatic steps are isentropic, and there is no temperature difference
between the heat reservoir and the working fluid in the heating/cooling steps.
400
The temperature difference between states
3 and 4 in Figure 5-5
is exaggerated to
make it clear that the
points are distinct. The
temperature change
in a pump is typically
closer to 1 to 2
degrees, rather than
15 to 20 degrees as the
figure implies.
350
Temperature (8C)
300
Furnace temperature
19
250
1
200
150
100
4
50
2
3
0
0
29
2
Ambient temperature
4
6
8
10
Specific entropy (kJ/kg-K)
Figure 5‑5 An example Rankine heat engine cycle, illustrated on the T-Ŝ plot for water.
State 1 is the steam leaving the boiler, 2 is the liquid/vapor mixture leaving the turbine, 3
is the liquid leaving the condenser, and 4 is the liquid leaving the pump. States 19 and 29
represent an alternative cycle in which the steam leaving the boiler is superheated rather
than saturated.
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196 Fundamentals of Chemical Engineering Thermodynamics
FOOd FOr
thOught
5-2
If the direction of the
Carnot cycle were
reversed, progressing
through the states in
the sequence 4-3-2-1-4
instead of 1-2-3-4-1,
what would it do?
In this discussion we
assume the fuel burns
at a specific, fixed
temperature, and thus
we treat T 5 270°C for
the high-temperature
reservoir as a design
constraint.
In their chapter on
“experience-based
principles” for design,
Turton et al. (Turton,
2009) recommend a
heat exchanger use a
minimum approach
temperature (ΔT) of
10°C for fluids above
ambient temperature, and a minimum
ΔT of 5°C for heat
exchangers that use
refrigeration.
Problem 5-6 asks you
to calculate values of
the heat and work in
each step of the Rankine engine described
by Figure 5-5.
Enthalpy of vaporization is defined as the
molar enthalpy of
saturated vapor minus
the molar enthalpy of
saturated liquid at the
same T and
P: ΔH vap 5 H V 2 HL
oVerall ProCeSS: ConTinuouS, STeaDy‑STaTe, anD CloSeD SySTem
Two prominent applications for heat engines are powering vehicles and generating
electricity. In such applications the engine is required to operate continuously for significant periods of time, so steady state is the most practical mode of operation. The
cycle is a closed system and the operating fluid is continuously circulated through
the following four steps.
Carnot Step 1: Isothermal Heating A reversible heat engine requires that the
heating of the operating fluid is carried out isothermally, and at essentially the same
temperature as the high-temperature reservoir, as shown in Figure 5-4. Practically
speaking, there must be a significant ΔT between the reservoir and the operating
fluid in order for the heat transfer to be accomplished in a heat exchanger of manageable size. Thus, if the high-temperature reservoir is a furnace at TH 5 270 8C (as in
Figure 5-5), the operating fluid would most likely leave the heat exchanger at ≈250 –
260 8C (250 8C is used in Figure 5-5). This observation still leaves the question: how
can a real process most closely mimic the isothermal heating of a real heat engine?
Normally, adding heat to a substance increases its temperature. As illustrated
in Figure 4-19, a gas can be heated isothermally if it expands at the same time. This
simultaneous heating and expansion is feasible in a closed, piston-cylinder type system. Here, we are considering a steady-state process with continuous circulation of
the working fluid. Practically speaking, it’s difficult to design one device that acts
both as a heat exchanger and as a pressure controller in a continuous, steady-state
operation.
Instead, the most straightforward way to heat something isothermally is through
a phase change, as in Example 4-13. A heat exchanger that took in saturated liquid
and emitted saturated vapor would be isothermal (assuming the pressure drop across
the heat exchanger was negligible).
Realistically, the boiler in a real Rankine heat engine doesn’t truly operate isothermally: the entering liquid is not saturated liquid and the exiting vapor is sometimes superheated, for reasons that are explained in the discussion of the remaining
steps. However, water has a large enthalpy of vaporization, so a boiler is certainly a
logical operation for adding a large amount of heat to the circulating liquid without
exceeding the limit imposed by the temperature of the heat source.
Carnot Step 2: Adiabatic, Reversible Expansion Since the goal of the design is to
convert heat into shaft work, the expansion is carried out in a turbine. In a Carnot
cycle this step is adiabatic and reversible. Real turbines are well approximated as
adiabatic but are not reversible, as discussed in Example 4-7. Since a detailed discussion of the design and construction of turbines is beyond the scope of this book,
here we simply note that the most effective heat engine will use turbines that are as
close to reversible as possible. “Close to reversible” for a turbine can be quantified
through the efficiency, as discussed in Section 4.4.2.
For the moment, assume the material entering the turbine is saturated vapor,
as illustrated by path 1-2 in Figure 5-5. The material exiting the turbine in such a
case is typically a liquid–vapor mixture, composed primarily of vapor with a small
amount of liquid entrained in it. If the liquid fraction in a turbine becomes too large,
the equipment is vulnerable to damage from erosion. Consequently, in designing a
heat engine, a maximum acceptable liquid fraction leaving the turbine would likely
be specified as a design constraint. If the predicted liquid fraction is unacceptably
large, one way to alter the design is to send superheated vapor, rather than saturated
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C H A P T E R 5 Thermodynamic Processes and Cycles
vapor, into the turbine. This alternative is illustrated by path 19 to 29 in Figure 5-5;
note that state 29 is nearer to the saturated vapor curve than state 2, meaning the
vapor fraction (q) is higher. If the entering steam were superheated to an even higher
temperature, the exiting steam could contain no liquid at all.
Carnot Step 3: Isothermal Cooling The cooling step is carried out in a condenser.
A real condenser operates at a higher temperature (40 8C in Figure 5-5) than the
low-temperature reservoir (20 8C), for the same reason that the boiler operates at a
lower temperature than the high-temperature reservoir: a significant temperature
difference is required to allow the desired heat transfer to occur in a heat exchanger
of manageable volume.
Figure 5-4 shows that, if a heat engine were truly reversible and water were the
operating fluid, the material exiting the condenser (state 3 in Figure 5-4) would be a
liquid–vapor mixture. This is necessary for the following reasons:
197
The quality (q) of a
liquid–vapor mixture
was defined in Section
2.2.5 as the fraction
of mass in the vapor
phase.
If any entropy change occurred in the pump, the pump wouldn’t be reversible.
Low-pressure saturated liquid has a lower specific entropy than high-pressure
saturated liquid.
Therefore, in order to have the same entropy as the high-pressure liquid, the
material entering the compression step cannot be saturated liquid; it must be a
liquid–vapor mixture.
However, it is easier and cheaper to design a pump to compress a pure liquid rather
than compressing a liquid–vapor mixture. Consequently, in the Rankine cycle, the
condenser is designed to produce a pure liquid, as practical and safety considerations are prioritized over the fractionally larger efficiency that might result from
compressing a liquid–vapor mixture. There is no practical advantage to further cooling the water once it has all condensed, so the condenser is designed to produce
saturated liquid.
Carnot Step 4: Adiabatic Compression The compression is carried out in a pump.
The work required to compress a pure liquid is comparatively small, and it is therefore normal that a large pressure increase is accompanied by only a small temperature
increase. Consequently, the liquid leaving the pump is subcooled liquid, as illustrated
by path 3 to 4 in Figure 5-5.
Thus, in general terms, the Rankine cycle illustrated in Figure 5-5 can be rationalized as “the closest thing to a Carnot heat engine that we can build after taking
real-world considerations into account.”
The next section examines the detailed quantitative design of a Rankine heat
engine.
Because vapors
typically have a
specific volume far
larger than liquids,
it typically takes far
more work to compress a vapor than to
compress an equivalent mass of liquid.
See Example 3-9
which examines a
pump.
5.2.2 complete design of a rankine heat engine
This section illustrates the design and analysis of a Rankine heat engine.
cOMPLete quantitative descriPtiOn OF a ranKine cycLe
examPle 5‑1
A Rankine heat engine is to be designed, with the following specifications and constraints:
The boiler will operate at a maximum T 5 200 8C.
The condenser will operate at a minimum T 5 100 8C.
The turbine has an efficiency of 75%.
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198 Fundamentals of Chemical Engineering Thermodynamics
The liquid fraction exiting the turbine cannot exceed 10%.
Water, in liquid and/or vapor form, is the operating fluid.
Determine the state of the water entering each of the four steps of the process in Figure 5-6, the efficiency of the heat engine, and the mass flow rate of water necessary for the
engine to produce 10 MW of net work.
QH
1
Boiler
T ≤ 2008C
4
WS,pump
In Chapters 1 through
4, step 1 in almost all
problems is “define a
system.” This chapter
explores larger processes that are made
up of several unit
operations. We will
often define several
different systems in
the course of solving
one problem.
WS,turb
5 0.75
3
Condenser
T ≥ 1008C
q ≥ 0.9
2
QC
Figure 5‑6 Schematic of the Rankine heat engine examined in Example 5-1.
An engineer designing a real heat engine
would use thermodynamics in combination
with economic considerations. Most
chemical engineering
programs include
one or more design
courses that cover
equipment sizing and
economics.
The work produced
by a turbine increases
as the pressure difference between inlet
and outlet increases.
This fact was mentioned as early as
Chapter 1, but Example 5-2 demonstrates it
quantitatively.
SOLUTION: This example is not fully specified; it requires us to make some decisions
and assumptions.
Step 1 Preliminary design decisions—boiler
Section 4.4.4 demonstrated that efficiency in a Carnot engine is given by:
Carnot 5 1 2
TC
TH
(5.1)
While the exact equation does not hold for real, irreversible heat engines, it provides a physical insight—that efficiency is maximized when TC is minimized and TH is
maximized—which is true for real heat engines. Consequently, we will assign the steam
exiting the boiler to be at T 5 2008C, which is the specified maximum temperature. We
cannot set the pressure of the steam exiting the boiler above the vapor pressure at 200 8C
(15.55 bar), or the liquid will not boil. We could set the pressure lower than 15.55 bar,
but we note that turbine work will be maximized if the pressure difference between the
entering and exiting steam is maximized. Consequently, we begin by assigning:
the steam exiting the boiler and entering the turbine (1) will be saturated steam
at T 5 200 8c.
Step 2 Preliminary design decisions—condenser
We wish the turbine outlet pressure to be as low as possible, so that the pressure drop
across the turbine is as large as possible. The lowest temperature at which the condenser
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C H A P T E R 5 Thermodynamic Processes and Cycles
can operate is 100 8C. The vapor pressure at this temperature, P 5 1.013 bar, is the lowest
pressure at which the vapor will condense. Consequently:
the water exiting the condenser and entering the pump (3) will be saturated liquid at T 5 100 8c.
Step 3 Simplifying assumptions
We were asked to find the state of the fluid leaving each of the four steps of the process.
We’ve already determined two. We can assume, as we did in our discussions of the processes in Figures 5-4 and 5-5, that the pressure of the fluid does not change in the boiler
or the condenser. Therefore, the steam exiting the turbine is at P 5 1.013 bar and the
water exiting the pump is at P 5 15.55 bar. We can apply entropy and energy balances to
determine more properties of these streams.
Step 4 Apply entropy and energy balances to reversible turbine
To determine the work produced in the turbine, we must apply the known turbine efficiency of 75%. First we consider a reversible turbine, which has the following entropy
balance (see Example 4-7):
Ŝout 2 Ŝin 5 Ŝ2, rev 2 Ŝ1 5 0
(5.2)
And any turbine, reversible or irreversible, has the following energy balance (see Section 3.6.3):
·
WS,turbine
(5.3)
5 Ĥout 2 Ĥin
·
m
Step 5 Collect data
kJ
. At T 5 1008C, saturated liquid and satukg ? K
kJ
kJ
rated vapor have ŜL 5 1.3072
and ŜV 5 7.3541
. Consequently, the material
kg ? K
kg ? K
leaving the reversible turbine must be a liquid–vapor mixture; its specific entropy falls
between ŜL and ŜV.
From Appendix A, Ŝ1 5 Ŝ2,rev 5 6.4302
199
FOOd FOr
thOught
5-3
Throughout this chapter we assume the
properties of a fluid do
not change as the fluid
is transported from
one unit operation to
the next. Is this true?
The state of a pure,
homogeneous phase
has two degrees of
freedom, as shown by
the Gibbs phase rule.
Thus, to describe the
“state” of a material,
one intensive property
(here, pressure) is not
a complete answer—
we need two.
In steps 4 through 8,
the system is a
reversible turbine.
We account for the
non-idealities in the
turbine using the
turbine efficiency.
Step 6 Compute q for reversible turbine
The mixture leaving the reversible turbine is 84.7% vapor, as
Ŝ2,rev 5 s1 2 qrevdŜ L 1 qrev Ŝ V
6.4302
1
2
(5.4)
1
kJ
kJ
kJ
5 s1 2 qrev d 1.3072
1 sqrev d 7.3541
kg ? K
kg ? K
kg ? K
2
qrev 5 0.847
Step 7 Compute specific enthalpy of stream leaving reversible turbine.
The specific enthalpy of the exiting stream can be determined as
Ĥ2,rev 5 s1 2 qrevd Ĥ L 1 sqrevdĤ V
1
Ĥ2,rev 5 s1 2 0.847d 419.2
2
1
(5.5)
2
kJ
kJ
kJ
1 s0.847d 2675.6
5 2330.8
kg
kg
kg
Step 8 Solve energy balance for reversible turbine
The energy balance (Equation 5.3) can now be solved to find the reversible work:
·
WS,turbine,rev
kJ
kJ
kJ
5 Ĥ2,rev 2 Ĥ1 5 2330.8
2 2792.0
5 2461.2
(5.6)
·
m
kg
kg
kg
pitFaLL
preventiOn
The symbol qrev is
used to emphasize
that this number is
specific to the reversible turbine. The
problem specified that
the turbine effluent
cannot exceed 10%
liquid. The result of
84.7% vapor and
15.3% liquid in step 6
does not violate that
constraint because the
reversible turbine is
purely hypothetical.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
200 Fundamentals of Chemical Engineering Thermodynamics
We use the efficiency
to transition our
analysis from the hypothetical reversible
turbine to the real
turbine.
Step 9 Apply turbine efficiency
The turbine is known to be 75% efficient. The turbine efficiency is defined as
·
WS, turbine,act
5 ·
WS, turbine,rev
and can be solved for the actual work:
·
·
WS, turbine, act sWS, turbine, rev d
kJ
kJ
5
5 s0.75d 2461.2
5 2345.9
·
·
m
m
kg
kg
1
In step 10, the system
is the REAL turbine.
If this calculation had
shown q < 0.9, we
could have lowered
the pressure of the
steam entering the
turbine (while leaving
the temperature unchanged). Example 5-2 further illustrates the cause and
effect relationships
among the inlet pressure, work, and conditions of the outlet
stream.
In step 11, the system
is the condenser.
Cogeneration is production of electricity
and useable heat in a
single facility. Here,
the shaft work could
be converted into
electricity, and the heat
removed by the condenser can be transferred to other uses
(e.g., space heating, hot
water for bathroom
and kitchen use, etc.).
In step 12, the system is
the entire heat engine.
(5.7)
2
(5.8)
Step 10 Determine q for fluid leaving real turbine
Now that we’ve analyzed the turbine, it seems natural to progress to the next step: the
condenser. However, we cannot forget the specification that the stream leaving the turbine can contain no more than 10% liquid; we need to verify that this is true. Applying
the energy balance to the actual turbine gives
·
WS, turbine, act
(5.9)
5 Ĥ2, act 2 Ĥ1
·
m
Find the specific enthalpy of the stream leaving the actual turbine:
·
WS,turbine,act
kJ
kJ
kJ
1 Ĥ1 5 2345.9
1 2792.0
5 2446.1
Ĥ2,act 5
·
m
kg
kg
kg
(5.10)
Now determine the actual liquid fraction of the material leaving the actual turbine:
Ĥ2,act 5 s1 2 qactdĤ L 1 sqactdĤ V
2446.1
1
2
1
kJ
kJ
kJ
1 sqactd 2675.6
5 s1 2 qactd 419.2
kg
kg
kg
(5.11)
2
qact 5 0.90
thus, the material leaving the turbine (2) is a mixture of 10% liquid and 90% vapor
at T 5 100 8c, and has a specific enthalpy of 2446.1 kj/kg.
The fraction of liquid in the turbine effluent is essentially at its maximum allowable value,
so we will assume the decisions we’ve made up to this point are acceptable and continue
with the design process.
Step 11 Find heat removed from condenser
The energy balance for one side of a heat exchanger, as derived in Section 3.6.4, is
·
Q
(5.12)
· 5 Ĥout 2 Ĥin 5 Ĥ3 2 Ĥ2
m
For this condenser, the leaving stream is saturated liquid at 100°C, and the entering stream
is the stream that exits the turbine, the specific enthalpy of which was found in step 10.
·
QC
kJ
kJ
kJ
(5.13)
· 5 Ĥ3 2 Ĥ2 5 419.2 kg 2 2446.1 kg 5 22026.9 kg
m
Step 12 Apply energy balance to entire heat engine
The entire heat engine is a closed system operating at steady state, so the energy balance is
·
·
05Q1W
(5.14)
But there are two distinct steps involving heat and two distinct steps involving work:
·
·
·
·
(5.15)
0 5 QC 1 QH 1 WS,turbine 1 WS,pump
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C H A P T E R 5 Thermodynamic Processes and Cycles
·
·
At this point QC and WS,turbine are known. If either the pump work or the boiler heat is
determined, the other can be found by closing this energy balance. We cannot calculate
·
·
either QH or WS,pump from an energy balance because we don’t have enough information
to determine the specific enthalpy of stream 4, so what alternatives are available?
Step 13 Calculate pump work
Refer to Example 3-8, which demonstrated that pump work is well approximated by
#
·
·
·
WS,pump 5 V dP < VsPout 2 Pind
(5.16)
in which liquid volume is modeled as constant. Here, stream 4 is leaving the pump and stream
3 is entering, and since we don’t know the mass flow rate, we divide both sides by it to get
·
WS,pump
(5.17)
5 V̂ sP4 2 P3d
·
m
·
WS,pump
m3
5 0.001043
s15.549 bar 2 1.013 bard
·
m
kg
1
2
1
Pa
5
3 10 bar
21
N
m2
1
Pa
21
1J
1N?m
1 kJ
kJ
5 1.52
211000
J2
kg
Notice that only 1.52 kJ/kg of work is required to compress the liquid from 1.013 bar to
15.549 bar. While the addition of energy in a pump results in a temperature rise, it is only
an incremental increase.
Step 14 Apply energy balance to pump
The energy balance for a pump mirrors the energy balance for a turbine:
·
WS,pump
5 Ĥout 2 Ĥin 5 Ĥ4 2 Ĥ3
·
m
(5.18)
Thus, the specific enthalpy of the water leaving the pump can be determined by
kJ
kJ
1.52
5 Ĥ4 2 419.2
kg
kg
Ĥ4 5 420.7
(5.19)
kJ
kg
Step 15 Describing the state of stream 4
We now have two physical properties of the pure water leaving the pump: P 5 15.549 bar
and Ĥ 5 420.7 kJ/kg. This means (according to Gibbs phase rule) all other intensive
properties of the stream must have unique values. If we were required to find the temperature, we could estimate it from data in Appendix A.
Step 16 Find heat added in boiler
We can now solve the energy balance for the entire heat engine, Equation 5.15 from step 12:
·
·
·
·
0 5 QC 1 QH 1 WS,turbine 1 WS,pump
·
·
·
·
QC QH WS,turbine WS,pump
05 · 1 · 1
1
m
m
m·
m·
·
QH
kJ
kJ
kJ
1 · 1 2345.9
1 1.52
(5.20)
0 5 22026.9
kg
m
kg
kg
·
QH
kJ
· 5 2372.7 kg
m
1
2
1
2
201
PitFaLL
PreventiOn
A common error is
modeling the liquid
leaving the pump as
saturated liquid. There
is no reason to expect
this liquid is saturated;
it is done just because
the properties of saturated liquids are available. But we will see
below how bad this
assumption is.
By definition, V 5 MV̂
We use the specific
volume of stream 3 in
solving Equation 5.17,
because it is available
in the steam tables.
The specific volume of
stream 4 is currently
unknown (but assumed to be approximately the same).
In step 14, the system
is the pump.
PitFaLL
PreventiOn
The specific enthalpy
of saturated liquid at
200°C is 852.3 kJ/kg.
Had we assumed the
material leaving the
pump was saturated
liquid, in effect, we
would have been
assuming the work
added in the pump was
over 430 kJ/kg (852.3419.04), when in reality
it is less than 2 kJ/kg.
The compressed liquid
table in Appendix A
contains data at P 5
10 bar and P 5 50 bar.
The compressed liquid
leaving the pump is at
P 5 15.549 bar.
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202 Fundamentals of Chemical Engineering Thermodynamics
Step 17 Find overall efficiency of heat engine
The overall efficiency is:
·
·
·
2sWS,turbine 1 WS,pumpd
2WS,net
kJ
kJ
1 1.52
2 2345.9
·
·
kg
kg
m
m
H.E 5
5
5
·
·
kJ
QH
QH
2372.7
kg
·
·
m
m
H.E. 5 0.145
1
Here “MW” is the
unit megawatts,
though the symbol
MW is also frequently
used for molecular
weight.
2
(5.21)
Step 18 Calculate mass flow rate
Finally, we see from step 17 that the net work produced is 344.4 kJ per kilogram of steam,
so the mass flow rate required to produce 10 MW is calculated as follows:
1
· 344.4 kJ
10 MW 5 m
kg
· 5
m
1
10 MW
kJ
344.4
kg
21
W
10
MW
6
21
J
s
1W
1
21
2
(5.22)
2
kg
1 kJ
5 28.9
s
1000 J
In Example 5-1 we determined the physical state of the material leaving each piece
of equipment, the heat and work duties in each piece of equipment, the mass flow
rate of H2O circulating through the process, and the overall efficiency of the heat
engine. This is as far as we can go using thermodynamics. The next step in designing a real heat engine would probably be detailed sizing of the individual pieces of
equipment, which is addressed in other courses in a typical chemical engineering
curriculum.
The preliminary design decisions made in Example 5-1 are not definitive. Changing the operating temperature in the condenser or boiler by a few degrees can have
a measurable impact on the economics of the process, since these temperatures are
design parameters that influence both the efficiency of the process and the cost of
the equipment. Problems 5-17 and 5-20 examine the economics of a Rankine heat
engine.
5.2.3 design variations in the rankine
heat engine
Section 5.2.2 illustrated a complete quantitative analysis of a Rankine heat engine,
and illustrated some design decisions (e.g., assigning outlet pressures to pump and
turbine). This section looks deeper into design considerations in the Rankine heat
engine.
Section 5.2.1 noted that excessive amounts of liquid entrained in the vapor
in a turbine can damage the equipment. In Example 5-1, the fraction of liquid
in the stream exiting the turbine (1 – q) was 0.1, which was the exact maximum
liquid fraction stated for that turbine. Suppose this liquid fraction had been 0.13.
What aspect of the design could we have changed to fix the problem? Example 5-2 explores quantitatively the effect of superheating the liquid coming out
of the boiler.
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C H A P T E R 5 Thermodynamic Processes and Cycles
saturated versus suPerheated steaM entering a turbine
A turbine (Figure 5-7) has an efficiency of 80% and an outlet pressure of P = 0.5 bar. The
inlet stream is at T = 300°C, and can be either a saturated steam or a superheated steam
at either P = 3 bar, 5 bar, 10 bar, or 30 bar.
T 5 3008C
5 0.8
P5?
examPle 5‑2
Recall we defined
quality (q) as the
vapor fraction, so a
maximum liquid fraction of 0.1 corresponds
to a minimum q of 0.9.
P 5 0.5 bar
q ≥ 0.9
WS 5 ?
Figure 5‑7 Turbine examined in Example 5-2.
A. Which of these inlet streams provides the most work without exceeding a maximum
allowable liquid fraction of 10% in the steam exiting the turbine?
B. What is the efficiency of a Rankine heat engine operating using this turbine, with the
inlet stream chosen in part A?
SOLUTION:
Step 1 Apply entropy and energy balances to reversible turbine
Define stream 1 as entering the turbine and stream 2 as exiting. As in Example 5-1, the
entropy balance for a reversible turbine is
Ŝ2, rev 2 Ŝ1 5 0
(5.23)
Ŝ2,rev 5 Ŝ1
and the energy balance for a turbine is
·
WS, turbine, rev
5 Ĥ2, rev 2 Ĥ1
·
m
(5.24)
Step 2 Determine condition of stream leaving reversible turbine.
Each of the five possible inlet streams has a different specific entropy, but they are all known,
and summarized in Table 5-1. At the outlet pressure P = 0.5 bar, the saturated liquid and
kJ
kJ
vapor specific entropies are ŜL 5 1.0912
and ŜV 5 7.5930
. Consequently,
kg ? K
kg ? K
if the exiting material is a liquid–vapor mixture, the fraction of liquid in the material
leaving the turbine can be found using
Ŝ2, rev 5 s1 2 qrev d ŜL 1 sqrevd ŜV
(5.25)
with Ŝ2,rev representing the specific entropy of the outlet stream, which is equal to the specific entropy of the inlet stream, as shown in Equation 5.23.
Step 3 Solve for reversible work
Once qrev is known, the outlet specific enthalpy can be determined using
Ĥ2, rev 5 s1 2 qrev d ĤL 1 sqrev dĤV
203
(5.26)
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204 Fundamentals of Chemical Engineering Thermodynamics
tabLe 5-1 Effect of inlet pressure on shaft work produced by turbine, for uniform
inlet temperature of 300°C.
1kgkj? K2
1kgkj 2
1kg2
kj
kj
·
Ws, rev
kg
2685.7
3069.6
–383.9
0.980
2598.6
3064.6
–466.0
7.1246
0.928
2479.2
3051.6
–572.4
30
6.5412
0.838
2272.4
2994.3
–721.9
85.8 (sat’d)
5.7059
0.710
1976.3
2749.6
–773.3
inlet
T (°c)
inlet
P (bar)
300
3
7.7037
1.017*
300
5
7.4614
300
10
300
300
Ŝ2, rev 5 Ŝ1
qrev
Ĥ2, rev
Ĥ1
1 2
*
For P = 3 bar, the result of qrev = 1.017 is obtained from Equation 5.25. The “vapor fraction” being greater than 1 is
physically unrealistic and simply tells that this is, in fact, NOT a liquid–vapor mixture. The fact that Ŝ2, rev is not between ŜL and ŜV indicates that the stream is not a mixture of liquid and vapor, but even if we didn’t notice this, we
can come to the same conclusion mathematically when we obtain an unrealistic result from Equation 5.25.
·
and WS,rev,turbine can be found from Equation 5.24. The results of steps 1 through 3 are summarized in Table 5-1.
kJ
from the P = 3 bar inlet stream
As it turns out, the specific entropy 7.7037
kg ? K
kJ
,
is slightly larger than that of superheated steam at 0.5 bar and 100°C 7.6953
kg ? K
and interpolating between the values for T = 100°C and 150°C yields a temperature of
101.7°C. This temperature is used in the determination of Ĥ2, rev and subsequent calculations. For the other four inlet streams, the outlet stream is indeed a VLE mixture and
equations 5.25 and 5.26 do model them accurately.
Overall, Table 5-1 shows that the work increases as the inlet pressure increases, as
expected. However, now we must determine which outlet streams exceed the allowable
10% liquid.
1
2
1
2
Step 4 Determine condition of stream exiting turbine
The actual work can be determined from the reversible work by applying the efficiency
of 80% for
·
WS,turbine,act
5 ·
5 0.8
(5.27)
WS,turbine,rev
FOOd FOr
thOught
5-4
In the last three rows
of Table 5-2, where the
exit stream is a VLE
mixture, we don’t actually need to know its
temperature to solve
this problem—but
how would we find T
if we needed to?
The actual specific enthalpy of the outlet stream can be found by solving the energy
balance:
·
WS,turbine,act
(5.28)
1 Ĥ1
Ĥ2,act 5
·
m
And the quality of the actual outlet stream is determined using Equation 5.26, as was
done for the reversible outlet stream. These results are summarized in Table 5-2. Notice
that in the case of the P 5 5 bar inlet stream, the actual outlet stream is actually a superheated steam, though it was a VLE mixture in the reversible case. For the higher inlet
pressures, the actual outlet stream is a VLE mixture, but with higher quality than the
corresponding reversible turbine. These observations can be understood by the fact that
the actual turbine removes less energy than does the reversible turbine.
ASIDE: This example presents a very plausible scenario. The temperature of the steam leaving the boiler is constrained by the temperature of whatever is being used as the heat
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C H A P T E R 5 Thermodynamic Processes and Cycles
tabLe 5-2 Effect of inlet pressure on physical state of outlet stream, for a turbine
with fixed inlet temperature (T 5 300°C and fixed outlet pressure
(P 5 0.5 bar).
1 2 W· 1kg2 Ĥ 1kgkj 2 state of exit stream
inlet
T (°c)
inlet
P (bar)
kj
·
Ws,rev
kg
300
3
300
kj
s,act
2,act
–383.9
–307.1
2762.5
superheated vapor,
T = 140.9°C
5
–466.0
–372.8
2691.8
Superheated vapor,
T = 104.8°C
300
10
–572.4
–457.9
2593.7
VLE mixture, q = 0.978
300
30
–721.9
–577.5
2416.8
VLE mixture, q = 0.901
300
85.8 (sat’d)
–773.3
–618.6
2131
205
A design specification
like “q must be greater
than 0.9” typically has
a safety margin built
into it, since fluctuations in process conditions can occur due
to process upsets or
during the unsteadystate operation associated with startup and
shutdown.
VLE mixture, q = 0.777
source, so all the variants we are examining have an inlet T = 300°C. Our goal is to obtain
the maximum possible work for this boiler outlet temperature. Our calculations reveal that
maximum work is obtained for a saturated steam (P = 85.8 bar) leaving the boiler, but that
this gives us an unacceptable amount of condensation in the turbine. Setting the turbine
inlet pressure to 30 bar solves this problem; q is now almost exactly 0.9. Further reducing
the turbine inlet pressure unnecessarily lowers the work produced by the turbine.
SOLUTION B:
Step 5 Apply energy balance to condenser
We now establish the specifications for a Rankine heat engine that incorporates the turbine selected in part A (inlet P 5 30 bar). The VLE mixture entering the condenser, as
shown in Table 5-2, has Ĥ2,act = 2416.8 kJ/kg. Saturated liquid at P = 0.5 bar has Ĥ =
340.5 kJ/kg, so the heat removed in the condenser is
·
QC
kJ
kJ
kJ
(5.29)
· 5 Ĥout 2 Ĥin 5 Ĥ3 2 Ĥ2 5 340.5 kg 2 2416.8 kg 5 22076.3 kg
m
Step 6 Determine pump work
The work added in the pump can be estimated, as in Example 3-8, using Equation 5.30 for
·
WS, pump
5 Vˆ dP < V̂sPout 2 Pind
(5.30)
·
m
#
·
WS, pump
m3
Pa
5 0.00103
s30 bar 2 0.5 bard 105
·
m
kg
bar
1
2
1
21
N
m2
1 Pa
1
21
1
J
N?m
21
1 kJ
1000 J
2
·
WS, pump
kJ
5 3.04
·
m
kg
Figure 5-5 illustrates
that compression of
a saturated liquid
doesn’t inherently lead
to a phase change,
as does expansion
of a saturated vapor.
Consequently, while
there exists a practical
incentive to consider
feeding superheated
vapor to the turbine,
there is no analogous
incentive to consider
subcooling the liquid
entering the pump.
The specific volume
used is for the inlet
condition, saturated
liquid at P 5 0.5 bar,
because the condition
of the outlet stream
isn’t fully specified.
Step 6 Apply energy balance to entire process
·
QH
The overall energy balance for the entire process is solved to find · using
m
·
·
·
·
QC QH WS, turbine WS, pump
05 · 1 · 1
1
·
·
m
m
m
m
(5.31)
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206 Fundamentals of Chemical Engineering Thermodynamics
·
kJ QH
kJ
kJ
0 5 22076.3
1 · 1 2577.5
1 3.04
kg
m
kg
kg
·
QH
kJ
· 5 2650.8 kg
m
1
2
Step 7 Determine overall efficiency of complete process
The overall efficiency of the entire cycle can be determined by
·
·
·
2sWS,turbine 1 WS,pump d
2WS,net
kJ
kJ
1 3.04
2 2577.5
·
·
kg
kg
m
m
H.E 5
5
5
·
·
kJ
QH
QH
2650.8
kg
·
·
m
m
H.E. 5 0.217
1
2
(5.32)
Example 5-2 illustrates the cause and effect relationship between the pressure of the
steam entering the turbine and the physical state of the stream exiting the turbine.
It further demonstrates the motivation for using superheated steam entering the
turbine, even though saturated steam may have the potential to produce more work.
Two other design variants in the Rankine cycle are worth mentioning:
If the steam entering the turbine is to be superheated, a separate boiler and superheater can be employed. The boiler and the superheater are two separate heat
exchangers; the first produces the steam and the second heats the steam to the desired turbine inlet temperature. The advantage of this approach is that separate heat
sources can be used. In the example illustrated in Figure 5-8b, the boiler heat source
can be ~2008C, while the superheater heat source must be above 4008C. Table 5-3
illustrates that heat becomes more expensive as the temperature at which it is delivered increases, so the scheme in Figure 5-8a, in which ALL of the heat is provided
from a source operating above 4008C, would not be favorable. The energy balance is
substantially the same whether the “boiler” is a single piece of equipment or two; the
absolute amounts of heat added in Figures 5-8a and b are the same.
Q 5 2499.9 kJ/kg
Boiler
Liquid water
P 5 10 bar
Ĥ 5 764.0 kJ/kg
Superheated steam
P 5 10 bar, T 5 4008C
Ĥ 5 3263.9 kJ/kg
(a)
Q 5 485.8 kJ/kg
Q 5 2014.1 kJ/kg
Boiler
Liquid water
P 5 10 bar
Ĥ 5 764.0 kJ/kg
Superheater
Saturated steam
P 5 10 bar
Ĥ 5 2778.1 kJ/kg
Superheated steam
P 5 10 bar, T 5 4008C
Ĥ 5 3263.9 kJ/kg
(b)
Figure 5‑8 Formation of steam at P = 10 bar and T = 4008C by (a) a single boiler
and (b) a boiler/superheater sequence.
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C H A P T E R 5 Thermodynamic Processes and Cycles
steam Pressure
(gage) (bar)
steam
temperature (°c)
utility cost ($/gj)
Low-pressure steam
5
160
14.05
Medium-pressure steam
10
184
14.83
High-pressure steam
41
254
17.70
heat source
Based on data from Turton, Bailie,
Whiting and Shaewitz, Analysis,
Synthesis and Design of Chemical
Processes, 3rd ed. Prentice-Hall, 2009.
tabLe 5-3 Estimates of the cost of steam heating, as a function of temperature
(Turton, 2009).
The single turbine can be replaced by multiple turbines with inter-stage heating. The
purpose of superheating the steam entering a turbine is to reduce the fraction of steam
that condenses in the turbine. It is possible to achieve the same effect by using two turbines with moderate pressure drops, rather than a single turbine with a large pressure
drop, as illustrated in Figure 5-9. Figure 5-9a is the single turbine and requires superheated steam at 350°C. Figure 5-9b begins with superheated steam at a lower temperature of 250°C, but heat is added between the two turbines. Because of the temperature
drop in the first turbine, it is possible for the inter-heater to operate at the same temperature (and use the same heat source) as the boiler, rather than operating at a higher
temperature as does the super-heater in Figure 5-9. The two systems produce similar
amounts of work, and both avoid significant amounts of liquid condensing in a turbine.
These design alternatives are further explored in Problem 5-15.
Superheated steam
T 5 2508C
P 5 10 bar
Ĥ 5 2942.6 kJ/kg
Superheated steam
T 5 3508C
P 5 10 bar
Ĥ 5 3157.7 kJ/kg
WS 5 2315.5 kJ/kg
VLE, P 5 2 bar, q 5 0.964
Ĥ 5 2627.1 kJ/kg
WS 5 2692.4 kJ/kg
Q 5 343.9 kJ/kg
Superheated, P 5 2 bar, T 5 2508C
Ĥ 5 2971.0 J/kg
VLE Mixture
T 5 69.18C
P 5 0.3 bar
q 5 0.931
WS 5 2366.2 kJ/kg
VLE, P 5 0.3 bar, q 5 0.991
Ĥ 5 2465.3 kJ/kg
(a)
(b)
Figure 5‑9 Comparison of (a) a single reversible turbine to (b) a sequence of two
reversible turbines with inter-stage heating. The overall pressure drop in both cases is
identical, and both designs maintain a liquid fraction (1-q) below 10% in the turbines.
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207
208 Fundamentals of Chemical Engineering Thermodynamics
5.3 refrigeration—the vapor-compression cycle
This section discusses design and analysis of refrigeration cycles. Section 5-1 showed
that the physical properties of water figure prominently in the design of a Rankine
heat engine. We needed to know properties (e.g., values of Ĥ and Ŝ) in order to perform the energy and entropy balances, but more fundamentally our design choices
were guided by the properties of water (particularly the relationship between pressure and temperature). We will find the same is true in designing refrigeration cycles.
While industrial manufacturing processes often employ refrigeration on a large
scale, we will begin by considering a household kitchen refrigerator, since that is
more likely familiar to the reader.
FOOd FOr
thOught
5-5
We used water as the
operating fluid for the
Rankine cycle; why
don’t we use water as
a refrigerant?
5.3.1 a household refrigerator
Household refrigerators are normally set to maintain a temperature at or below
40°F. For simplicity, let’s assume the interior temperature is 41°F, which is 5°C. The
ambient temperature may vary throughout the day and year; for this example let’s
assume its 68°F, or 20°C. In effect, the refrigerator is a machine that makes a nonspontaneous process occur: heat is transferred from the cool inside (41°F) to the
warmer surroundings (68°F). This section describes the classic process by which
refrigerators operate: the vapor-compression cycle.
Figure 5-10 shows a schematic of the vapor-compression cycle, in which a refrigerant is circulated through a continuous, steady-state process.
QH
High P
Condenser
WS,comp
Throttling
valve
Low P
High P
Boiler
Compressor
Low P
QC
Figure 5‑10 Schematic of the vapor-compression
refrigeration cycle.
In the boiler, heat is transferred to the refrigerant, causing the refrigerant to boil.
The refrigerant, now a vapor, enters a compressor, which increases its pressure.
The high-pressure vapor refrigerant enters a condenser, where heat is transferred to the surroundings, causing the refrigerant to condense to liquid.
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C H A P T E R 5 Thermodynamic Processes and Cycles
209
The high-pressure liquid refrigerant enters a throttling valve, where its pressure
and temperature drop.
The low-pressure refrigerant enters the boiler.
For this to work, the refrigerant must have a temperature below 41°F when it contacts the inside of the refrigerator (here we assign it to 32°F = 0°C) and must have
a temperature above 68°F when it contacts the ambient air (here we will set it to
77°F = 25°C). Frequently, the contents of the refrigerator, and the surroundings, can
each be modeled as heat reservoirs, and that is what we will do in this section. Note
the following to analyze a refrigeration cycle.
·
QC will represent the rate of heat transfer from the cold reservoir.
·
QH will represent the rate of heat transfer to the hot reservoir.
These definitions mirror the definitions used for heat engines, but in a refrigera·
·
tor, QC will be positive and QH will be negative.
Like the Rankine heat engine, the vapor-compression cycle makes use of the physical phenomenon of a phase change to transfer significant quantities of heat. Thus,
the refrigerant must boil at 32°F and then condense at 77°F. The only way to cause
this dramatic change in boiling temperature is a pressure change. Consequently, the
compressor and the valve are used to control the pressure.
One question that might arise is “Why not place a turbine, instead of a valve, after the
condenser? That way some useful work could be obtained while the necessary pressure
drop occurs.” While this is a logical thought, it is instructive to recall Examples 3-8 and 4-8.
These illustrate that the work required to compress a liquid is typically very small compared to the work required to compress an equivalent mass of vapor to the same pressure.
The inverse is also true: expansion of a typical liquid in a turbine produces an insignificant
amount of work compared to a vapor expanding from the same initial pressure to the
same final pressure. Compared to a valve, a turbine is more expensive, more complex, and
more likely to require maintenance and repair, and the work it would provide in a typical
vapor-compression cycle is not enough for the turbine to be cost-effective.
vaPOr-cOMpressiOn cycLe
examPle 5‑3
The contents of a refrigerator are at 5°C. The surroundings are at 20°C. Heat transfers
through the walls to the inside of the refrigerator at a rate of 100 kJ/min. Consequently, the
refrigeration process operates continuously at steady state, and removes 100 kJ/min of heat,
thus allowing the contents to be maintained at a constant temperature of 5°C (Figure 5-11).
The refrigerant is Freon® 22, and thermodynamic properties are provided in Appendix F. The material leaving the boiler is saturated vapor at 0°C and the material leaving
the condenser is saturated liquid at 25°C. The compressor has an efficiency of 75%. Find
the refrigerant flow rate required to remove 100 kJ/min of heat from the inside of the
refrigerator, and the rate at which the compressor requires work.
In order to design and
build a refrigerator,
you’d need to determine the size of each
piece of equipment,
the diameter of the
pipes, etc. While these
are not directly “thermodynamics” questions, the flow rate of
coolant is an essential
parameter that informs these design
decisions.
SOLUTION: The 100 kJ/min removed from the contents of the refrigerator by our defi-
·
nition is QC.
Step 1 Overall energy balance around entire process
Like the Rankine heat engine, each individual step in the vapor-compression process is
an open system, but the refrigeration process as a whole is a closed system that operates
at a steady state. Thus, the energy balance simplifies to
·
·
05Q1W
(5.33)
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
210 Fundamentals of Chemical Engineering Thermodynamics
Freon® is a registered
trademark of DuPont.
Several different
refrigerants (distinguished from each
other by a following
number) have been
marketed under the
trade name Freon.
Tambient 5 208C
QH
Saturated
liquid
T 5 258C
3
Condenser
4
WS,comp
In step 1, the system is
the entire cycle.
1
Low P
Boiler
5 0.75
Saturated
vapor
T 5 08C
2
QC 5 100 kJ/min
T 5 58C
Figure 5‑11 Vapor-compression cycle described in
Example 5-3.
FOOd FOr
thOught
5-6
Since we have saturated liquid entering
the valve, and the
valve adds neither
heat nor work, why
can’t we assume the
exiting stream is also
saturated liquid?
In steps 2 through 4,
the system is a
hypothetical reversible compressor with
exactly the same inlet
stream, and the same
outlet pressure, as the
real compressor.
Heat transfer occurs in the evaporator and condenser, but no work occurs in these steps.
Shaft work is added in the compressor, but typical compressors are reasonably modeled
as adiabatic (see Section 3.6.3). The throttling valve changes the pressure of the refrigerant without transferring work or heat. Consequently, the energy balance can be written
more specifically as:
·
·
·
0 5 QC 1 QH 1 WS
(5.34)
·
·
·
Thus, if we can find any two of QC, QH, and WS, we can compute the other through
Equation 5.34.
·
There’s no immediate way to find QH—we know exactly what’s coming out of the
condenser but not what’s going in. Similarly, we know exactly what’s coming out of the
boiler (saturated vapor at 0°C), but we don’t know exactly what’s going in. However, we
can determine the compressor work by applying the known efficiency.
Step 2 Apply energy and entropy balances to reversible compressor
Section 3.6.3 derived the energy balance for a compressor as
·
WS,compressor
5 Ĥout 2 Ĥin 5 Ĥ3 2 Ĥ2
·
m
(5.35)
which is valid whether the compressor is reversible or not. Because we know the vapor
entering the compressor is saturated, Ĥ2 can be found from Appendix F and is 405 kJ/
kg. However, at this point, we don’t have any information on the vapor leaving the
compressor.
This is a steady-state process, so the accumulation term in the entropy balance is
zero. Thus,
·
· Ŝ 2 m
· Ŝ 1 Q 1 S·
05m
(5.36)
in in
out out
gen
T
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C H A P T E R 5 Thermodynamic Processes and Cycles
·
Compressors, like pumps and turbines, are reasonably modeled as adiabatic, so Q = 0.
·
For the particular case of a reversible compressor, Sgen is also zero, leaving
· Ŝ 2 m· Ŝ
(5.37)
5m
0 5 m· Ŝ 2 m· Ŝ
in
in
out
out,rev
2
2
3
3,rev
At this point, m· is unknown, so even though we have to find it eventually, it makes sense
to divide it out:
Ŝ3,rev 5 Ŝ2
211
Because this is a
steady-state process,
and there is only one
stream entering
and one leaving,
· 5m
· .
m
in
out
(5.38)
Step 3 Determine specific enthalpy of vapor leaving reversible compressor
The vapor leaving the boiler is saturated vapor at T = 0°C; which, according to Appendix F,
kJ
has Ŝ < 1.755
. We know from Equation 5.38 that Ŝ for the vapor leaving the comkg ? K
pressor is the same. We need a second property of the exiting stream in order to find Ĥ3
from Appendix F. We get this by considering the design of the remainder of the process.
The condenser produces saturated liquid at T 5 25°C, which according to Appendix F
must have P < 10.5 bar. Assuming the pressure does not change significantly in the condenser, the vapor leaving the compressor also has P = 10.5 bar.
kJ
According to Appendix F, vapor with P 5 10.5 bar and Ŝ 5 1.755
has
kg
?K
kJ
Ĥ 5 425 . This is Ĥ3,rev, which is the specific enthalpy of the gas leaving our hypothetikg
cal reversible compressor.
Step 4 Solve energy balance for reversible compressor
We can now determine the shaft work required for the reversible compressor as
·
WS, compressor,rev
5 Ĥ3,rev 2 Ĥ2
·
m
Graphically, this step
can be carried out by
following the lines of
constant entropy on
the P-H diagram from
T = 0°C to T = 25°C.
(5.39)
·
WS,compressor,rev
kJ
kJ
kJ
5 425
2 405
5 20
·
m
kg
kg
kg
Step 5 Calculate actual work required for compressor
The definition of efficiency for a compressor is:
·
WS,rev
compressor 5 ·
WS,act
(5.40)
PitFaLL
PreventiOn
Applying the known efficiency of 75% and the result of step 4 gives
kJ
·
20
WS,act
kg
kJ
· 5 0.75 5 26.7 kg
m
(5.41)
Step 6 Determine specific enthalpy of ACTUAL vapor leaving compressor
We now know the work required to run the compressor, but still need to know the actual
specific enthalpy of the stream leaving the compressor, so that we can analyze the condenser. Thus, we now apply the energy balance to the ACTUAL compressor:
·
WS,compressor, act
5 Ĥout,act 2 Ĥin 5 Ĥ3,act 2 Ĥ2
(5.42)
·
m
which can be solved using
·
WS,compressor,act
kJ
kJ
kJ
5 405
1 26.7
5 431.7
(5.43)
Ĥ3,act 5 Ĥ2 1
·
m
kg
kg
kg
A careless error at this
point would be ignoring units and substitut·
ing QC = 100 and
·
WS = 20 into the overall energy balance
(Equation 5.34) to
·
solve for QH. We cannot add or subtract
numbers until we have
them in the same units.
In step 7, the system
is the ACTUAL compressor, not the reversible one we analyzed
in Steps 2-4.
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212 Fundamentals of Chemical Engineering Thermodynamics
In step 8, the system
is the condenser; specifically the side of the
heat exchanger that
has the refrigerant
flowing through it.
Step 8 Apply energy balance to condenser
The condenser is a heat exchanger, and the energy balance (see Section 3.6.4) is
·
QH
· 5 Ĥout 2 Ĥin 5 Ĥ4 2 Ĥ3
m
Ĥ3 is known from step 7, and the stream leaving the condenser (4) is saturated liquid, so
its specific enthalpy can be found in Appendix F. Thus,
·
QH
kJ
kJ
kJ
(5.45)
· 5 Ĥ4 2 Ĥ3 5 230 kg 2 431.7 kg 5 2201.7 kg
m
·
Step 9 Solve energy balance to find QC
·
·
The work (WS) and the condenser heat (QH) are both now known—not on an absolute basis but in energy per unit mass. Consequently, the energy balance for the
whole system (Equation 5.34) can be solved if it is divided through by the mass
flow rate:
·
·
·
QC QH WS
05 · 1 · 1 ·
(5.46)
m
m
m
·
which can be solved for QC by
·
·
·
QC 2QH 2WS
kJ
kJ
kJ
5 201.7
1 226.7
5 175
· 5 m
· 1 m
·
m
kg
kg
kg
1
FOOd FOr
thOught
5-7
How would you characterize the overall
effectiveness of this
cycle with a single
number, similar to the
efficiency of the heat
engine?
(5.44)
2 1
2
(5.47)
Step 10 Find mass flow rate
The heat added to the boiler was given as 100 kJ/min, so Equation 5.47 can be solved
for m· as
·
QC
kJ
(5.48)
· 5 175 kg
m
kJ
kg
min
· 5
5 0.571
m
kJ
min
175
kg
100
Step 11 Calculate rate at which compressor uses work
In step 6, we found the work required in the compressor for each kilogram of refrigerant. Now that we know the mass flow rate, we can find the rate at which work is added
in absolute terms:
·
WS, act
kJ
(5.49)
· 5 26.7 kg
m
1
2 1
·
· 26.7 kJ 5 0.571 kg
WS, act 5 m
kg
min
kJ
2 126.7 kgkJ 2 5 15.2 min
Example 5-3 presents quantitative modeling of a vapor-compression cycle.
Notice that there was very little mention of the throttling valve. The energy balance
for the typical valve (see Section 3.6.1) is 0 5 Ĥin 2 Ĥout. Thus, in Example 5-3 (and
Figure 5-11), Ĥ1 5 Ĥ4 5 230 kJykg, and the valve unit operation makes no explicit
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C H A P T E R 5 Thermodynamic Processes and Cycles
contribution to the overall energy balance (Equation 5.34) for the cycle. While it
wasn’t necessary to solve the problem, we can quantify the state of the fluid leaving
the valve knowing that Ĥ1 5 230kJykg and P1 = P2 = 5 bar. From Appendix F, it is a
liquid–vapor mixture with q = 0.15. The fluid entering the valve is saturated liquid.
The pressure drop causes some of the liquid to evaporate, and the energy required
for evaporation results in a decrease in the temperature of the fluid.
5.3.2 coefficient of performance
The purpose of refrigeration is to maintain a space below ambient temperature by
removing heat from that space. The effectiveness of the cycle is quantified by the
coefficient of performance (C.O.P.):
·
QC
C. O. P. 5 ·
(5.50)
WS
·
where QC is the rate at which heat is removed from the contents of the refrigerator
·
and WS is the rate at which work is added. This definition reflects the fact that we
·
wish to maximize QC (removing this heat is the whole point of the process) and
·
we wish to minimize WS (since this work represents the expense required for operating the process).
Why is it called the “coefficient of performance” rather than the “efficiency”?
Because conventionally, “efficiency” is a fraction; its value ranges from 0 to 1 (or 0 to
100%). The efficiency of a heat engine is always a fraction: specifically the fraction
of entering heat that is converted into useful work (see Equation 5.21 and Exa·
mple 4-11). However, Examples 5-3 and 5-4 show that QC can be, and often is, larger
·
than WS. Consequently, the word “efficiency” is not used, but the “coefficient of performance” serves exactly the same purpose as the “efficiency” of a heat engine: It
summarizes the overall effectiveness of the entire process in a single numerical value.
5.3.3 the carnot refrigerator
Section 4.4.4 introduced the Carnot heat engine, which is an idealized heat engine
that is 100% reversible and follows four steps.
Isothermal heating
Adiabatic expansion
Isothermal cooling
Adiabatic compression
213
The coefficient of
performance of a
refrigeration cycle
is analogous to the
efficiency of a heat
engine—they are both
single numbers that
benchmark the overall
effectiveness of the
process.
The coefficient of
performance for the
refrigeration cycle in
Example 5-3 was
(100 kJ/min)/(15.2 kJ/
min) = 6.57.
·
QH does not appear
in the coefficient of
performance equation,
since it represents
energy expelled to
the surroundings and
likely has little cost or
practical value.
Like the Carnot heat
engine, the Carnot
refrigerator is an
idealized limit that
can never be attained
in the real world, but
serves as a useful
point of comparison
for real refrigerators.
If these four steps were executed in the opposite order, they would mirror the steps
in the vapor-compression cycle introduced in Section 5.3.1:
Adiabatic compression occurs in the compressor
Isothermal cooling occurs in the condenser
Adiabatic expansion occurs in the valve
Isothermal heating occurs in the boiler
A reversible cycle following these steps is called a Carnot refrigerator, as examined
in Example 5-4.
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214 Fundamentals of Chemical Engineering Thermodynamics
examPle 5‑4
a carnOt reFrigeratOr
A refrigeration process is designed to operate at steady state, removing 1000 kJ/min
of heat from a low-temperature reservoir at T = 41°F and expel the heat to a hightemperature reservoir at T = 68°F. If the cycle is reversible, what is its coefficient of
performance?
SOLUTION: Note the question, as asked, gives no specific information about the process: what is used as a refrigerant, etc. Just like the reversible heat engine in Example
4-11, we will find we can analyze a reversible refrigerator knowing nothing other than the
fact that it’s reversible and the temperatures of the two heat reservoirs between which
it operates.
·
In order to calculate the coefficient of performance, we need to find two things: QC
·
and WS.
High-temperature reservoir
T 5 688F
QH 5 ?
WS 5 ?
A conventional refrigeration process only
has one step involving
work, but if there were
·
more than one, W in
this equation would
simply represent the
NET work.
FOOd FOr
thOught
5-8
Why didn’t we do an
overall entropy balance in Example 5-3?
For heat transfer to
be reversible, the
temperature difference driving the heat
transfer must be
negligible. This means
the temperature of
the heat reservoir is
essentially identical to
the temperature of the
system at the boundary where the heat
transfer occurs.
Q C 5 1000 kJ/min
Low-temperature reservoir
T 5 418F
Figure 5‑12 Reversible refrigerator described in Example 5-4.
Step 1 Apply energy balance to entire process
We are explicitly told the process is designed to operate at steady state. We are not explicitly told that the cycle is a closed system, but this is a reasonable assumption based on
what we’ve learned about cycles up to this point. Thus, there is no accumulation of energy
in the system, and the only terms that appear in the energy balance are heat and work:
·
·
·
0 5 QC 1 QH 1 WS
(5.51)
·
·
Again, we have two distinct terms, QC and QH, representing heat exchange with each of
·
the two heat reservoirs. QC is given, so we have two unknowns, and need one additional
·
equation to solve for WS.
Step 2 Apply entropy balance to entire process
Again, the accumulation term is 0 for this steady-state process, and there is no material enter·
ing or leaving the system. The process is reversible, so Sgen=0. The entropy balance simplifies to:
·
·
QC QH
05
1
(5.52)
TC
TH
·
Since QH does not appear in the coefficient of performance equation, we solve Equa·
tion 5.52 for QH, so that we can eliminate it from the energy balance. Thus,
·
2QCTH
·
(5.53)
QH 5
TC
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
C H A P T E R 5 Thermodynamic Processes and Cycles
Step 3 Solve energy balance
Substituting Equation 5.53 into Equation 5.51 gives
·
2QCTH
·
·
1 WS
0 5 QC 1
TC
1
2
·
· TH
WS 5 QC
21
TC
(5.54)
Step 4 Evaluate coefficient of performance
· ·
By definition the coefficient of performance is QC /WS. Using Equation 5.54 allows us to
relate coefficient of performance to the temperatures of the heat reservoirs:
·
·
QC
QC
1
C. O. P. 5 · 5
5
(5.55)
T
WS
· TH
H
21
QC
21
TC
TC
1
C.O.P. 5
2
1 21 2
1
TH
TC
21
TC
TC
5
TC
TH 2 TC
Step 5 Introduce numerical values for TC and TH
In quantifying the C.O.P., we need to convert temperature to an absolute scale. Since the
given information is in Fahrenheit, we convert it to the Rankine temperature scale:
C.O.P. 5
41 1 459.78R
5 18.5
s68 1 459.78Rd 2 s41 1 459.78Rd
(5.56)
215
The overall energy
and entropy balances
for a refrigeration
cycle are not substantially different from
the balances for a heat
engine, but the signs
·
· ·
of QC, QH, and WS are
opposites in the two
processes.
FOOd FOr
thOught
5-9
Why is the C.O.P. of
6.57 for the real
refrigerator in Example 5-3 so much lower
than that of a Carnot
refrigerator that operates between the same
temperatures?
Equation 5.55 is valid for any reversible refrigeration process. Again, the reversible
process is an idealized, unattainable limit, but Equation 5.55 provides some useful
insight into the performance of real refrigerators. Thus,
C.O.P. 5
TC
TH 2 TC
(5.57)
The lower the refrigerator temperature (TC), the lower the coefficient of
performance.
The bigger the temperature difference between the ambient surroundings (TH)
and the refrigerator temperature, the lower the coefficient of performance.
5.3.4 analyzing a refrigeration cycle with simple Models
Previous examples have examined water as the operating fluid in heat engines and
Freon® 22 as the operating fluid in a refrigerator. Both of these are compounds for
which complete data is available at the conditions of interest. However, we cannot
assume such data will always be available. For example, in the 1970s, household refrigerators used chlorofluorocarbons (CFCs) like Freon® 22 as refrigerants. In the
1980s, it was determined that these compounds were contributing to the depletion
of the atmospheric ozone layer, and in a relatively short amount of time, a new generation of refrigerants was developed and adopted, though this development and
refinement process is on-going even today. When one is developing or working with
new chemical products, one cannot expect that comprehensive data is going to be
available. The motivational example for Chapter 8 discusses this further. In Example 5-5, we examine a problem in which data for enthalpy and entropy is not available, so we have to estimate needed properties using ideal gas models.
Equation 5.57 is valid
only for a Carnot
refrigerator.
The phasing out of
CFCs and the development of new refrigerants is discussed
again in the Motivational Example of
Chapter 8.
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216 Fundamentals of Chemical Engineering Thermodynamics
examPle 5‑5
an ideaL gas reFrigerant
A refrigerator operates on the vapor-compression cycle as follows (see Figure 5-13).
The boiler produces saturated vapor at T = 285 K and P = 0.2 bar.
The compressor has an efficiency of 70%.
The condenser produces saturated liquid at T = 305 K and P = 0.9 bar.
QH
Saturated
liquid
P 5 0.9 bar
T 5 305 K
3
Condenser
4
WS,compressor
1
Boiler
5 0.7
Saturated
vapor
P 5 0.2 bar
T 5 285 K
2
QC
Figure 5‑13 Refrigeration cycle examined in Example 5-5.
The refrigerant has MW = 100 g/mol, ĈP* = 2 J/g · K, and ΔĤvap = 250 J/g. Both heat
capacity and enthalpy of vaporization can be assumed constant over the range of conditions in this process, and ideal gas behavior can be assumed at pressures below 1.0 bar.
Estimate the coefficient of performance.
SOLUTION:
The system in step 1
is the entire refrigeration cycle.
The system in
steps 2 and 3 is the
compressor.
Step 1 Overall energy balance for the entire process
As in Example 5-3, the overall energy balance for the refrigerator is:
·
·
·
(5.58)
0 5 QC 1 QH 1 WS,compressor
· ·
·
So if we know any two of QC, QH, and WS,compressor, we can find the third. As in Example 5-3,
we start with the compressor work, since the known efficiency gives us a starting point in
analyzing the compressor.
Step 2 Apply energy balance to compressor
As in Example 5-3, the energy balance for a compressor is
·
WS, compressor
5 Ĥout 2 Ĥin 5 Ĥ3 2 Ĥ2
m·
(5.59)
Here we cannot look up the specific enthalpies. Instead we can calculate the change in
enthalpy by applying the heat capacity.
This can also be
written dH = CP* dT
when CP* is expressed
on a molar basis.
Step 3 Relate change in enthalpy to change in temperature
As shown in Section 2.3.3, for any ideal gas process, we have
dĤ 5 ĈP* dT
(5.60)
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C H A P T E R 5 Thermodynamic Processes and Cycles
Because Ĉp* is constant, Equation 5.60 here integrates to
·
WS, compressor
5 Ĥ3 2 Ĥ2 5 ĈP*sT3 2 T2d
m·
(5.61)
We cannot progress further without knowing the temperature change that occurs in the
compressor. There is no immediate way to find this for the real compressor, but we can
assess it using the entropy balance for the reversible compressor.
Step 4 Apply entropy balance to a reversible compressor
As in Example 5-3, the compressor is an adiabatic, steady-state process, so if it is assumed
to also be reversible, the entropy balance simplifies to
Ŝ3,rev 2 Ŝ2 5 0
ln
T3, rev
T3,rev
P2
2
3,rev
1 T 2 1 R ln 1P 2 5 0
1T 2
5
2
(5.63)
P3, rev
R
ln
*
CP
P2
1 2
Step 5 Solve for outlet temperature from reversible compressor
The only unknown in Equation 5.63 is T3,rev, so it can be found: Note that heat capacity
must be converted from mass to molar basis, so the units are consistent with R.
ln
T3,rev
1285 K2
5
1
J
mol ? K
J
2
g?K
8.314
2
1
2
1
1 mol
0.9 bar
3 ln
100 g
0.2 bar
2
(5.64)
T3,rev 5 303.4 K
Step 6 Find work for reversible compressor
Applying Equation 5.61 to the reversible compressor gives
·
WS,comp,rev
J
J
5 ĈP*sT3, rev 2 T2d 5 2
s303.4 K 2 285 Kd 5 36.8
·
g
m
g?K
1
2
Equation 5.61 can be
applied to either the
real compressor or a
reversible compressor; we will need to do
both.
(5.62)
This is an ideal gas that has a constant heat capacity; the exact situation for which Equation 4.59 was derived. Applying the equation to this process:
Ŝ3,rev 2 Ŝ2 5 CP* ln
217
(5.65)
In steps 4 through 6,
the system is a hypothetical reversible
compressor with the
same inlet stream and
outlet pressure as the
real compressor.
FOOd FOr
thOught
5-10
Vapor pressure increases with increasing
temperature, but the
specific numerical
values vary drastically
from compound to
compound. Based on
Equation 5.63, what
kind of vapor pressure
vs. temperature relationship would make
for a good refrigerant?
Step 7 Apply definition of efficiency of compressor
Knowing that the compressor efficiency is 70%, we can find the actual compressor work
to be
·
WS,rev
(5.66)
compressor 5 ·
WS,act
J
·
WS,act 36.8 g
J
5
5 52.6
·
g
m
0.7
Step 8 Find temperature of gas leaving compressor
Having modeled the compressor, it is logical to move on to the condenser. However, first
we’d like to know exactly what is entering the condenser. We already know the pressure
of the gas is 0.9 bar, and the temperature can be found by applying Equation 5.61 to the
ACTUAL compressor for
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
218 Fundamentals of Chemical Engineering Thermodynamics
·
WS,comp,act
5 ĈP*sT3,act 2 T2d
m·
PitFaLL
PreventiOn
The temperature
303.4 K was specific
to the hypothetical
reversible compressor,
and cannot be used to
describe the ACTUAL
stream entering the
condenser.
In Step 9, the system
is the side of the condenser with refrigerant flowing through it.
PitFaLL
PreventiOn
You cannot simply
set Ĥ4 2 Ĥ3 equal
to CP*(T4 – T3). That
would be valid if the
material entering and
leaving were both
ideal gas, but in reality
the exiting stream is
liquid.
1
2
J
J
sT3,act 2 285 Kd
52.6 5 2
g
g?K
T3,act 5 311.3 K
Step 9 Apply energy balance to condenser
As in Example 5-3, the energy balance for the condenser is
·
QH
5 Ĥout 2 Ĥin 5 Ĥ4 2 Ĥ3
m·
(5.68)
What is Ĥ4 2 Ĥ3? Stream 3 is a gas at P = 0.9 bar and T = 311.3 K; stream 4 is a saturated
liquid at T = 305 K. Since Ĥ is a state property, we can evaluate the change in enthalpy
for any path from state 3 to state 4 and use the result in Equation 5.68.
Step 10 Construct two-step path
Using the methodology illustrated in Figure 5-14, we define state 3* as saturated vapor at
T = 305 K. We can introduce the specific enthalpy of this saturated vapor into Equation
5.68 as
·
QH
· 5 Ĥ4 2 Ĥ3 5 sĤ4 2 Ĥ3*d 1 sĤ3* 2 Ĥ3d
m
Gas
P 5 0.9 bar
T 5 311.3 K
Ĥ 3
Saturated vapor
T 5 305 K
Ĥ 3*
DĤ A
QH
m
PitFaLL
PreventiOn
(5.67)
DĤ B
(5.69)
Saturated liquid
T 5 305 K
Ĥ 4
5 Ĥ 4 2 Ĥ 3 5 DĤ A 1 DĤ B
Figure 5‑14 Decomposition of the condenser into two steps so that
change in specific enthalpy can be determined.
Sign errors often
occur when one
applies enthalpy of
vaporization (or
sublimation or fusion).
Keep in mind
saturated vapors are
higher in enthalpy
than saturated liquids
at the same conditions.
Since state 4 is liquid
and state 3* vapor,
Ĥ4 2 Ĥ3* MUST be a
negative number.
Meanwhile Ĥ3* 2 Ĥ3 represents the change in enthalpy when the ideal gas is cooled from
311.3 K to 305 K and can be related to temperature through Equation 2.33:
·
QH
*
(5.71)
· 5 2DĤvap 1 ĈP sT3* 2 T3 d
m
dĤ = ĈP* dT for any
ideal gas, as first discussed in Section 2.3.3.
Step 11 Solve for heat removed in condenser
Inserting known values into Equation 5.71 gives
·
QH
J
J
J
5 2250 1 2
s305 K 2 311.3 Kd 5 2262.6
·
g
g
m
g?K
The quantity Ĥ4 2 Ĥ3* is the change in enthalpy when saturated vapor turns to saturated
liquid; this is the opposite of the enthalpy of vaporization.
·
QH
(5.70)
5 2DĤvap 1 sĤ3* 2 Ĥ3d
m·
1
2
(5.72)
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C H A P T E R 5 Thermodynamic Processes and Cycles
Step 12 Solve overall energy balance
The mass flow rate is unknown but is uniform throughout the process, so we can divide
· for
the energy balance through by m
·
·
·
QC QH WS
05 · 1 · 1 ·
(5.73)
m
m
m
which allows us to find the heat in the boiler:
·
QC
J
J
1 52.6
0 5 · 1 2262.6
g
g
m
1
2
(5.74)
219
Efficiency and coefficient of performance
are dimensionless
quantities. In computing them, we can use
work and heat on an
absolute basis (J), rate
(J/s), or a mass (J/kg)
or molar (J/mol) basis,
but the units must
cancel.
·
QC
J
5 210.0
·
g
m
Step 13 Evaluate coefficient of performance
Applying the definition of coefficient of performance gives
·
QC
J
·
210
·
QC
g
m
C.O.P. 5 ·
5 3.99
5
5 ·
J
WS,comp,act WS,comp,act
52.6
g
·
m
(5.75)
5.4 Liquefaction
This chapter examines applications in which several individual steps are combined
into a complete process; specifically to this point a heat engine or a refrigeration
cycle. In all examples presented in Sections 5-2 and 5-3, the process as a whole was
a closed system; the operating fluid was continuously re-circulated. In this section,
we consider liquefaction, which is an open-system application for the purpose of
producing a chemical product in the liquid phase.
When we think of compounds like N2, O2, CO2, and CH4, we think of them as
gases. Nitrogen, for example, has a normal boiling point of 77.2 K, and thus exists
only as a gas in our everyday experience. However, liquid nitrogen has practical
applications as an extreme low-temperature coolant. For example, extreme cold
can be used to destroy growths and tissues that are unsightly (warts) or potentially malignant (tumors). Liquids also have the merit of being much denser than
gases; the Motivational Example in Chapter 7 discusses liquefying natural gas for
shipping.
Consequently, in this section, we design processes that liquefy a compound that
has an extremely low boiling point, primarily using nitrogen as an example. Before
reading further, consider the question: How many different ways can you think of to
make liquid nitrogen?
5.4.1 simple Liquefaction of nitrogen
The most straightforward way to condense a gas is to cool it below its boiling point.
However, in the case of nitrogen at atmospheric pressure, this would require a coolant that had a temperature below 77 K (2196°C). We could also consider compressing the nitrogen beyond its vapor pressure, but nitrogen has a critical point of
Liquefaction is a very
different application
than a heat engine or
a refrigeration process
in terms of its purpose,
but we will see it is
similar in that understanding the physical
and chemical properties of the compound
is essential for making
good design decisions.
A common application of liquid nitrogen
is its use to eliminate
warts.
The purpose of
making liquid nitrogen is to use it as an
ultra-low-temperature
coolant; it would be
self-defeating to use
up an ultra-lowtemperature coolant
in order to make
liquid nitrogen.
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220 Fundamentals of Chemical Engineering Thermodynamics
T = 126 K and P = 34 bar; thus, the liquid state is unattainable at temperatures
above 126 K, regardless of pressure.
Examining the data in Figure 2-3 reveals another possibility. The enthalpy of
supercritical nitrogen at T = 135 K and P = 6.4 MPa (64 bar) is about 170 kJ/kg.
Meanwhile, at P = 1 bar (approximately atmospheric pressure), saturated
liquid nitrogen has Ĥ 5 28 kJykg and saturated vapor has Ĥ 5 228 kJykg. While
the temperature of this equilibrium mixture is not shown explicitly in the figure,
it is quite close to the normal boiling point of 77 K. Example 5-6 illustrates a
straightforward approach for producing liquid nitrogen that is based upon these
observations.
examPle 5‑6
There is nothing
“special” about the
conditions T = 135 K
and P = 64 bar; they
are simply convenient
numbers to use for
this example. Supercritical nitrogen with
any initial state that is
located directly above
the two-phase region
in Figure 2-3 can be
expanded to produce
liquid nitrogen.
a siMPLe LiqueFac tiOn PrOcess
100 kg/min of supercritical nitrogen at T = 135 K and P = 64 bar flows into a chamber, as
illustrated in Figure 5-15. The sudden increase in diameter leads to a large pressure drop;
the pressure in the chamber is P = 1 bar. The chamber has no moving parts, operates at
steady state, and can be modeled as perfectly insulated. Find the flow rates of nitrogen
liquid and vapor leaving the chamber.
Nitrogen vapor
mout,vapor 5 ?
min 5 100 kg/min
T 5 135 K
P 5 64 bar
P 5 1 bar
Nitrogen liquid
mout,liquid 5 ?
Figure 5‑15 Schematic of simple liquefaction process.
PitFaLL
PreventiOn
In many examples,
there is only one
stream entering and
one stream leaving the
system. Here there are
two exiting streams, so
we can’t simply equate
· 5m
· .
m
in
out
The importance of
this process is that it
allows us to produce
liquid nitrogen at
≈77 K without
needing to start with
a coolant that is
below T = 77 K.
SOLUTION:
Step 1 Define a system
The chamber is the system.
Step 2 Apply mass balance to system
This is a steady-state process, so the accumulation term is 0. There is one stream entering
the system and two leaving (liquid and vapor), so the mass balance is
· 2m
·
·
05m
2m
(5.76)
in
out,liq
out,vap
Step 3 Apply energy balance to system
The accumulation term is again 0. There are no moving parts and the system is perfectly
·
·
insulated, so Q = W = 0. Thus, the only forms in which energy enters or leaves the system
are in material flows:
· Ĥ 2 m
·
·
Ĥ
Ĥ
(5.77)
2m
05m
in
in
out,liq
out,liq
out,vap
out,vap
Step 4 Insert known information and data
The entering mass flow rate is given, and according to Figure 2-3, the specific enthalpy of
superheated nitrogen at T = 135 K and P = 64 bar is 170 kJ/kg.
The exiting streams are at P = 1 bar. At this pressure, saturated liquid has
Ĥ 5 28 kJykg and saturated vapor has Ĥ 5 228 kJykg. Inserting this information into
the energy balance (Equation 5.77) gives
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C H A P T E R 5 Thermodynamic Processes and Cycles
1
0 5 100
kg
min
2 1170 kg2 2 m· 128 kg2 2 m· 1228 kg2
kJ
kJ
out,liq
kJ
out,vap
221
(5.78)
And the mass balance (Equation 5.76) becomes
0 5 100
kg
·
·
2m
2m
out,vap
out,liq
min
(5.79)
·
·
and m
,
Equations 5.78 and 5.79 are thus two equations in two unknowns; m
out,liq
out,vap
which are exactly the two quantities we are trying to determine. The result is
kg
·
5 29.0
m
out,liq
min
kg
·
5 71.0
m
out,vap
min
Example 5-6 shows that we can produce pure liquid nitrogen at T ≈ 77 K (≈ 2196°C),
if we first produce supercritical nitrogen at P = 64 bar and T = 135 K. The example
raises two questions.
1. What’s the most efficient way to produce supercritical nitrogen at P = 64 bar and
T = 135 K?
2. The process in Example 5-6 only liquefies 29% of the entering nitrogen. What
should be done with the 71 kg/min of vapor product? Is there a more efficient
method that will liquefy a larger fraction of the entering nitrogen?
The following two sections examine these issues.
5.4.2 energy-efficient compression processes
The preceding section demonstrates that we can produce liquid nitrogen at atmospheric pressure (T ≈ 77 K) if we start with a supercritical fluid that has a lower specific
enthalpy than that of saturated nitrogen vapor. In Example 5-7, we examine the compression of an ideal gas to the same conditions seen in Example 5-6: P = 64 bar and
T = 135 K. It is clearly more accurate to use real data rather than modeling nitrogen
as an ideal gas, and Example 5-8 does use data. However, the ideal gas model used in
Example 5-7 will serve as a simpler way to illustrate some key physical phenomena.
cOMpressiOn OF an ideaL gas
An ideal gas has C = (7/2)R. We are designing a steady-state process to compress the
gas from an initial state of P = 1 bar and T = 300 K to a final state of P = 64 bar and
T = 135 K. Find the work added and heat removed per mole of gas for each of the following processes (summarized in Figure 5-16).
*
P
A. The gas is compressed to P = 64 bar in an adiabatic, reversible compressor and then
enters a heat exchanger in which it is cooled to T = 135 K.
B. The gas is cooled to T = 135 K in a heat exchanger, compressed to P = 64 bar in
an adiabatic reversible compressor, and in a second heat exchanger, is cooled to
T = 135 K.
examPle 5‑7
It isn’t realistic that a
gas would behave
like an ideal gas at
P = 64 bar, but this
example is primarily
intended to illustrate the
differences between
three approaches to
designing the
compression process.
C. The gas is compressed in two separate adiabatic, reversible compressors. The first
has inlet P = 1 bar and outlet P = 8 bar, and the second compresses the gas from
P = 8 bar to P = 64 bar. Each compressor is preceded by a heat exchanger that
cools the gas to T = 135 K, and a third heat exchanger cools the final product to
T = 135 K.
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222 Fundamentals of Chemical Engineering Thermodynamics
WS,A 5 ?
P 5 1 bar
T 5 300 K
QA 5 ?
P 5 64 bar
T5?
P 5 64 bar
T 5 135 K
(a)
QB1 5 ?
P 5 1 bar
T 5 300 K
WS,B 5 ?
P 5 1 bar
T 5 135 K
QB2 5 ?
P 5 64 bar
T5?
P 5 64 bar
T 5 135 K
(b)
QC1 5 ?
P 5 1 bar
T 5 300 K
WS,C1 5 ?
P 5 1 bar
T 5 135 K
QC2 5 ?
P 5 8 bar
T5?
WS,C2 5 ?
P 5 8 bar
T 5 135 K
QC3 5 ?
P 5 64 bar
T5?
P 5 64 bar
T 5 135 K
(c)
Figure 5‑16 Three methods of compressing and cooling an ideal gas to P = 64 bar and
T = 135 K.
In steps 1 through 3,
the system is the
compressor. We can·
not find Q in the heat
exchanger without
knowing the temperature of the stream
entering it, which we
will learn by modeling
the compressor first.
SOLUTION A:
Step 1 Apply energy and entropy balances to compressor
This is an adiabatic, steady-state compressor, so the energy balance we first saw in
Section 3.6.3 applies. For this problem it is most convenient to write it on a molar basis:
·
WS
(5.80)
5 H out 2 H in
n·
The entropy balance for the steady-state, adiabatic, reversible compressor that was established in Example 4-8 is
S out 2 S in 5 0
(5.81)
Step 2 Relate molar entropy to temperature and pressure
The gas in this problem is an ideal gas with constant heat capacity, which is the exact situation for which Equation 4.59 was derived. Applying that equation to this compressor
The compressors in
this problem are
assumed to be reversible because this is a
simplified example
intended to illustrate
specific physical
phenomena. The
compressors in
Examples 5-8 and 5-9
are modeled more
realistically.
S out 2 S in 5 0 5 C*P ln
05
1 2 1
1 T 2 1 R ln 1P 2
(5.82)
2
(5.83)
Tout
Pin
in
out
1
Tout
7
1 bar
R ln
1 R ln
2
300 K
64 bar
2
Tout 5 984 K
Step 3 Relate molar enthalpy to temperature
For any ideal gas, dH = CP* dT. Because CP* is constant, the change in molar enthalpy in
Equation 5.80 becomes:
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C H A P T E R 5 Thermodynamic Processes and Cycles
·
WS,A
5 H out 2 H in 5 CP* sTout 2 Tind
n·
223
(5.84)
·
WS, A 7
J
5 8.314
s984 K 2 300 Kd
n·
2
mol ? K
1
2
·
WS, A
J
5 19,915
n·
mol
Step 4 Apply energy balance to heat exchanger
The energy balance for one side of a heat exchanger, as derived in Section 3.6.4, is
·
Q
5 H out 2 H in
(5.85)
n·
Step 5 Relate molar enthalpy to temperature
As in step 3, the change in molar enthalpy can be related to temperature:
·
Q
5 H out 2 H in 5 C*PsTout 2 Tind
n·
In steps 4 through 5,
the system is the heat
exchanger that follows
the compressor.
(5.86)
The temperature entering the heat exchanger is the same as the temperature leaving
the compressor, and the heat exchanger is designed to reduce the temperature to 135 K.
·
QA
7
J
J
8.314
s135 K 2 984 Kd 5 224,717
(5.87)
5
n·
2
mol ? K
mol
1 21
2
SOLUTION B: Now we turn to the case with one compressor and two heat exchangers.
Since we know exactly what is entering and leaving the first heat exchanger, that is a
simple place to start.
Step 6 Model heat exchanger before compressor
The ideal gas is cooled from 300 K to 135 K before entering the compressor. Applying
Equation 5.86 to this cooling process:
·
QB1
7
J
J
8.314
s135 K 2 300 Kd 5 24801
(5.88)
5 C*PsTout 2 Tin d 5
n·
2
mol ? K
mol
1 21
2
Step 7 Model compressor for part B
The compressor in part B can be modeled in the same way as the one in part A,
with the only difference being that Tin is now 135 K instead of 300 K. Applying
Equation 5.82 gives
05
172 R2 ln 1135 K2 1 R ln 1641 bar
bar 2
Tout
(5.89)
Tout 5 443 K
and applying Equation 5.84 to the new inlet and outlet temperature gives
·
WS,B 7
J
J
·n 5 2 8.314 mol ? K s443 K 2 135 Kd 5 8962 mol
1
2
Step 8 Model heat exchanger that follows compressor
Applying Equation 5.86 to the second heat exchanger gives
·
QB2
7
J
J
5
8.314
s135 K 2 443 Kd 5 28962
n·
2
mol ? K
mol
1 21
2
(5.90)
(5.91)
PitFaLL
PreventiOn
·
·
W in step 7 and Q
in step 8 are equal
in magnitude and
opposite in sign,
because enthalpy is
only a function of
temperature for an
ideal gas. The temperature changes in
steps 7 and 8 also are
equal in magnitude.
·
For a real gas, W and
·
Q would not be equal
in magnitude, since
the pressure change
occurring in the compressor would also
affect enthalpy.
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224 Fundamentals of Chemical Engineering Thermodynamics
Equation 5.82 reveals
that the work required
in a compressor is
primarily a function of
the compression ratio
Pout∙Pin, rather than
the absolute values of
Pout and Pin. Consequently, staged compressors are normally
designed with identical compression ratios:
8/1 and 64/8.
COMPARE PARTS A AND B: The results reveal that far less work was required to compress the colder (T = 135 K) gas than the warmer gas (T = 300 K). Physically, the result
can be understood because low-temperature gases are denser than high-temperature:
work is related to the change in volume, and is therefore smaller when the initial and
final volumes are both made smaller. Mathematically, the outcome can be understood
by examination of Equations 5.82 and 5.84. In both processes, Tout/Tin is identical and approximately equal to 3.3, but when the values of Tin and Tout are decreased in magnitude,
the difference Tout – Tin also decreases.
Work is further decreased when the compression is accomplished in stages, as shown
in part C.
SOLUTION C: Apply Equations 5.84 and 5.86 to two compressors and three heat
exchangers.
The analysis of the individual compressors and heat exchangers in part C mirrors
that in parts A and B and will not be shown in detail. Each compressor has a compression
ratio Pout/Pin = 8 and an inlet temperature of 135 K, identical outlet temperatures of 245 K,
and identical work required of 3188 J/mol. Table 5-4 presents a summary of the results
and a more detailed solution is available in the electronic supplements.
Table 5‑4 Comparison of three different approaches to compression.
FOOd FOr
thOught
5-11
Would a three- or fourstage compression
sequence further
reduce the work? Is
there any disadvantage
to adding more stages?
Example 5-7 incorporates several heat
exchangers that
operate at T = 135 K.
The coolant in these
heat exchangers is not
identified, but may
well be produced by a
separate liquefaction
process.
shaft work
required (j/mol)
heat removal
required (j/mol)
Part A: Single compressor
19,915
–24,717
Part B: Single compressor with
pre-cooling
8962
–4801 – 8962 = –13,763
3188 + 3188 = 6376
–4801 – 3188 – 3188 = –11,177
Part C: Two-stage compressor with
pre-cooling and inter-stage cooling
While we can’t expect a real compound to exhibit ideal gas behavior at P = 64 bar,
the solution to Example 5-7 reveals a strategy that informs our design of a complete
process for liquefaction of nitrogen. Liquefaction requires formation of a supercritical fluid at a high pressure and low temperature. Specifically, in Example 5-6, we used
nitrogen at P = 64 bar and T = 135 K. Thus, the process will not work at all unless we
have available a method of cooling the gas to 135 K. Rather than doing this once immediately before the expansion step, we minimize the needed work (and subsequent
heat removal) by cooling the nitrogen to the lowest attainable temperature prior to
compression. Example 5-8 examines the design of a complete liquefaction process for
nitrogen that integrates a two-stage compressor analogous to the one in Example 5-7c.
5.4.3 Linde Liquefaction
A complete liquefaction process for nitrogen must combine the elements of
Examples 5-6 and 5-7. First a series of compressors and heat exchangers is used
to produce supercritical nitrogen at P 5 64 bar and T 5 135 K. If this nitrogen is
expanded to a pressure of 1 bar, Example 5-6 reveals that <29% of the entering
nitrogen is condensed. The remainder of the nitrogen leaves the expansion as vapor
at T < 77 K and P 5 1 bar. It likely seems natural to recycle this nitrogen vapor to
the beginning of the process, and this is done in Example 5-8.
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C H A P T E R 5 Thermodynamic Processes and Cycles
cOMPLete PrOcess FOr LiqueFac tiOn OF nitrOgen
examPle 5‑8
100 kg/min of liquid nitrogen is produced by the steady-state process shown in Figure 5-17.
QA
P1 5 1 bar
T1 5 300 K
WS,A 5 ?
WS,B 5 ?
QB
HX A
QC
HX B
P2 5 1 bar
T2 5 135 K
P3 5 8 bar
T3 5 ?
HX C
P4 5 8 bar
T4 5 135 K
P5 5 64 bar
T5 5 ?
P6 5 64 bar
T6 5 135 K
Saturated vapor, P7 5 1 bar
100 kg/min saturated
liquid, P8 5 1 bar
225
Flash
P 5 1 bar
Figure 5‑17 Schematic of liquefaction process for Example 5-8.
1. Nitrogen enters the process at P = 1 bar and T = 300 K.
2. The nitrogen is cooled in a heat exchanger (A) to T = 135 K.
3. The nitrogen is compressed from P = 1 bar to P = 8 bar in one compressor (A), and
from P = 8 bar to P = 64 bar in a second compressor (B). Each compression step is
followed by a heat exchanger that cools the nitrogen to T = 135 K.
4. The nitrogen at P = 64 bar and T = 135 K enters an adiabatic flash vessel in which P
= 1 bar.
5. The liquid leaving the vessel is removed as product: saturated liquid nitrogen at P = 1 bar.
6. The vapor leaving the vessel is recycled to the heat exchanger described in step 2,
where it mixes with the fresh nitrogen feed.
Assuming both compressors have an efficiency of 80% and no pressure drop occurs in
any heat exchanger, find the total rate at which work is added to each of the two compressors. How much work is required for each kilogram of liquid nitrogen produced?
SOLUTION: The question focuses on the compressors, since work is a major expense in
the process. So we will start by examining the compressors, but will quickly realize that a
broader look at the process is needed.
Step 1 Apply energy balance to a compressor.
The energy balance here is substantially identical to Equation 5.80 in Example 5-7; but
here we express it on a mass basis since Figure 2-3, the thermodynamic diagram for nitrogen, is on a mass basis.
·
WS
(5.92)
· 5 Ĥout 2 Ĥin
m
This energy balance equation can be applied to both compressors, and this is done
explicitly in step 5. However, the equation immediately reveals a challenge: We cannot
solve for shaft work unless the mass flow rate through the compressors is known. We
know the product flow rate is 100 kg/min, but because of the presence of the recycle
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226 Fundamentals of Chemical Engineering Thermodynamics
stream, there is more than 100 kg/min of material travelling through the compressors.
Thus, we start by formulating a strategy to find all mass flow rates.
Step 2 Apply material balance to entire process
This is a steady-state process. If we draw our system around the entire process, there is
only one stream entering (1) and only one stream leaving (8). Consequently, the material
balance simplifies to
· 2m
·
05m
(5.93)
1
8
· 5 100 kg
· 5m
m
8
1
min
Step 3 Apply material balances to individual unit operations
The two compressors, and the heat exchangers that follow them (exchangers B and C),
each have only one inlet stream and one outlet stream, so the material balances for each
of these pieces of equipment simplify to min = mout. Consequently,
· 5m
· 5m
· 5m
· 5m
·
m
(5.94)
2
3
4
5
6
Finally, the flash vessel has one entering stream but two exiting streams:
· 2m
· 2m
·
05m
6
7
8
(5.95)
Or, applying the known value of the product flow rate gives
100
The mass balance
we would get from
heat exchanger A is
· 1m
· 2m
· ,
05m
1
7
2
which is essentially
identical to Equation
· 5m
· .
5.96, because m
2
6
PitFaLL
PreventiOn
The presence of a
recycle stream has a
huge effect on flow
rates throughout a
system. If we had used
the product flow rate
of 100 kg/min as the
flow rate through each
unit operation, our
calculated W in step 7
would be <30% of the
correct value.
The flow rate entering
and leaving both
compressors is equal
to m· 6 as shown in
Equation 5.94.
kg
·
· 2m
5m
7
6
min
(5.96)
Thus, Equation 5.96 is one equation in two unknowns. We need a second equation that
relates m· 6 to m· 7, and can obtain it using an energy balance around the flash vessel.
Step 4 Apply energy balance to flash vessel
This is a steady-state process in which energy only enters and leaves through material streams:
· Ĥ 2 m
· Ĥ 2 m
· Ĥ
(5.97)
05m
6
7
6
7
8
8
Stream 6 is at P = 64 bar and T = 135 K, and streams 7 and 8 are saturated vapor and
liquid at P = 1 bar. Thus, the state of each is fully known and specific enthalpies can be
obtained from Figure 2-3:
1
2
1
2 1
· 228 kJ 2 100 kg
· 170 kJ 2 m
05m
7
6
kg
kg
min
2 128 kgkJ 2
(5.98)
Simultaneous solution of Equations 5.96 and 5.98 gives
· 5 344.8 kg
m
6
min
· 5 244.8 kg
m
7
min
(5.99)
We can now analyze the compressors, knowing that the flow rate through each is
344.8 kg/min.
Step 5 Apply energy balance equation to each compressor
Applying Equation 5.92 to each of the two compressors and using the nomenclature of
Figure 5-17 gives
·
WS, A
(5.100)
· 5 Ĥ3 2 Ĥ2
m
6
·
WS, B
· 5 Ĥ5 2 Ĥ4
m
(5.101)
6
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C H A P T E R 5 Thermodynamic Processes and Cycles
227
The state of the nitrogen entering each compressor is known. From Figure 2-3:
Ĥ2 5 288
kJ
sP 5 1 bar, T 5 135 Kd
kg
Ĥ4 5 282
kJ
sP 5 8 bar, T 5 135 Kd
kg
(5.102)
The conditions of the exit streams cannot be determined directly; we must model the
compressors as reversible and then apply the known efficiency.
Step 6 Apply entropy balance to hypothetical reversible compressor
The entropy balance for a steady-state, adiabatic, reversible compressor, as established
in Example 5-7, is
Ŝout,rev 2 Ŝin 5 0
(5.103)
Here we don’t have an equation that relates entropy to T and P (as we did in Example 5-7);
better yet, we have real data in graphical form. Equation 5.103 can be applied by locating
the conditions of the entering stream on the graph and then following a line of constant
entropy upward to the outlet pressure. Once this point has been located, the specific enthalpy (or any other property) can be obtained.
Ĥ3,rev 5 410
kJ
sP 5 8 bard
kg
Ĥ5,rev 5 390
kJ
sP 5 64 bard
kg
(5.104)
In Example 5-7, with
an ideal gas, the values
of work in the two
compression stages
were identical. Here
they are similar, but
not identical, because
in a real gas, enthalpy
is a function of pressure in addition to
temperature.
Step 7 Solve energy balances for reversible compressors
Applying the specific enthalpy values that have been determined for compressor A gives
·
WS, A, rev
kJ
5 Ĥ3, rev 2 Ĥ2 5 410 2 288
(5.105)
·
m6
kg
1
kg
·
WS, A, rev 5 344.8
min
kJ
2 1122 kgkJ 2 5 42,069 min
The same process for compressor B gives:
1
kg
·
· sĤ
WS, B, rev 5 m
2 Ĥ4 d 5 344.8
6
5, rev
min
21
390
2
kJ
kJ
kJ
2 282
5 37, 241
kg
kg
min
(5.106)
FOOd FOr
thOught
5-12
The heat duties in the
heat exchangers were
not asked in the problem statement, but if
they were, how would
you find them?
Step 8 Apply known compressor efficiency
By definition (see Section 4.4.2), the efficiency of a compressor is
·
WS,rev
5 ·
WS,act
Thus, the actual work for each compressor can be obtained by applying the known
efficiency:
·
WS, A 5
·
WS, B 5
kJ
min
kJ
5 52, 586
0.8
min
42,069
37, 241
0.8
kJ
min
5 46, 552
(5.107)
kJ
min
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228 Fundamentals of Chemical Engineering Thermodynamics
The total work required, per kilogram of liquid nitrogen produced, is
kJ
·
WS,tot 52, 586 1 46, 552 min
kJ
5 991
· 5
kg
m
kg
8
100
min
This process is named
after a German
engineer named Carl
von Linde, who also
founded the company
that is now known as
The Linde Group.
examPle 5‑9
Example 5-9 makes
good use of the “cold”
vapor stream within the
same process that produced it. Sometimes,
when “in-process” recycle isn’t possible, one
can make use of “hot”
or “cold” streams for
heat exchange in other
processes at the same
facility.
(5.108)
Example 5-8 illustrates a complete liquefaction process that would work, but misses
an opportunity for improved efficiency. See if you can spot it before reading further.
In chemical processes, there are frequently steps at which heat must be added or
removed. Thus, when a stream is at either extremely high or extremely low temperature, we should think of it as a valuable resource. While producing nitrogen vapor
at 77 K is not the purpose of the liquefaction process, this vapor is nonetheless a
resource, and it is badly used in Example 5-8. Instead of simply recycling the cold
vapor to the front of the process, we can use it to pre-cool the superheated nitrogen
entering the flash vessel. This is the characteristic step of the Linde liquefaction
process. Compared to Example 5-8, this requires only one additional piece of equipment, and Example 5-9 illustrates the benefits.
Linde LiqueFac tiOn OF nitrOgen
100 kg/min of liquid nitrogen is produced by the following steady-state process shown
in Figure 5-18.
QA
P1 5 1 bar
T1 5 300 K
WS,A 5 ?
WS,B 5 ?
QB
HX A
QC
HX B
P2 5 1 bar
T2 5 135 K
P3 5 8 bar
T3 5 ?
HX C
P4 5 8 bar
T4 5 135 K
Sat. vapor, P8 5 1 bar
Flash
P 5 1 bar
100 kg/min sat.
liquid, P9 5 1 bar
P5 5 64 bar
T5 5 ?
P6 5 64 bar
T6 5 135 K
P10 5 1 bar
T10 5 120 K
HX D
P7 5 64 bar
T7 5 ?
System boundary for step 2
Figure 5‑18 Schematic of the Linde liquefaction process employed in Example 5-9.
1. Nitrogen enters the process at P = 1 bar and T = 300 K.
2. The nitrogen is cooled in a heat exchanger (A) to T = 135 K.
3. The nitrogen is compressed from P = 1 bar to P = 8 bar in one compressor (A), and
from P = 8 bar to P = 64 bar in a second compressor (B). Each compression step is
followed by a heat exchanger that cools the nitrogen to T = 135 K.
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C H A P T E R 5 Thermodynamic Processes and Cycles
4. The superheated nitrogen at P = 64 bar and T = 135 K enters another heat
exchanger (D) and is cooled further.
5. The nitrogen from step 4 enters an adiabatic flash vessel that is maintained at
P = 1 bar.
6. The liquid leaving the flash vessel is removed as product: saturated liquid nitrogen at
P = 1 bar.
7. The vapor leaving the vessel is used as the coolant for the heat exchanger (D) described
in step 4 and leaves this heat exchanger at T = 120 K.
8. The vapor is then recycled to the heat exchanger described in step 2, where it mixes
with the fresh nitrogen feed.
Assuming both compressors have an efficiency of 80% and no pressure drop
occurs in any heat exchanger, find the total rate at which work is added to each of the
two compressors. How much work is required for each kilogram of liquid nitrogen
produced?
SOLUTION:
Step 1 Apply mass balances throughout process
If we draw our system boundaries around the entire process, there is, like in Example 5-8,
only one stream entering and one stream leaving. Thus,
· 2m
·
05m
(5.109)
1
9
· 5 100 kg
· 5m
m
9
1
min
Also as in Example 5-8, the two compressors and heat exchangers B and C all have a
single inlet and single outlet stream, so at steady state, these flow rates are all identical. Heat exchanger D has two inlet streams and two outlet streams, but the streams
exchange heat without exchanging mass; the mass flow rates on both sides are unchanged.
Consequently,
· 5m
· 5m
· 5m
· 5m
· 5m
·
m
(5.110)
2
3
4
5
6
7
·
· 5m
m
10
8
(5.111)
Finally, applying a mass balance to the flash chamber gives
· 2m
· 2m
·
05m
7
8
9
(5.112)
Applying the known flow rate of stream 9, the product stream is
100
kg
·
· 2m
5m
8
7
min
(5.113)
As in Example 5-8, the material balance in Equation 5.113 reveals that we need one
more equation, so an energy balance is the logical next step. But an energy balance for
which unit? In Example 5-8, we used an energy balance around the flash vessel, but in
that problem, the temperature of the stream entering the flash was known. Here T7 is
unknown, and as a result, an energy balance around the flash chamber would give us an
additional equation but also an additional unknown ( Ĥ7 ).
Step 2 Energy balance around two unit operations
If we define a system containing both the flash vessel and heat exchanger D, shown in
Figure 5-18, the temperature and pressure of all entering and exiting streams are known,
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229
230 Fundamentals of Chemical Engineering Thermodynamics
and stream 7, for which T is unknown, is inside the boundaries of the system. The energy
balance for this system is
· Ĥ 2 m
· Ĥ 2 m
· Ĥ
(5.114)
05m
6
6
9
9
10
10
Specific enthalpies for streams 6, 9, and 10 can be obtained from Figure 2-3, and the flow
rate of stream 9 is also known. Thus,
1
2 1
· 170 kJ 2 100 kg
05m
6
kg
min
In Example 5-8, the
fraction of nitrogen
entering the flash step
that actually condensed
was 29%. Here, its 100
kg/min/237.9 kg/min =
42%, because the
temperature of the
nitrogen entering the
chamber is lower.
The total shaft work,
68,400 kJ/min, is
equivalent to 1140 kW
or 1529 hp.
2 128 kgkJ 2 2 m· 1273 kgkJ 2
10
(5.115)
This can be combined with Equation 5.113 to produce two equations in two unknowns,
· 5m
· and m
· 5m
· . Thus,
because in step 1, it was shown that m
6
7
8
10
100
kg
·
· 2m
5m
10
6
min
(5.116)
The simultaneous solution of Equations 5.113 and 5.116 is
· 5 237.9 kg
m
6
min
· 5 137.9 kg
m
10
min
(5.117)
Step 3 Apply entropy balances to reversible compressors
The compressors A and B are essentially identical in function to those in Example 5-8;
though the flow rate of nitrogen is lower, the inlet temperatures and pressures are the
same. Consequently, as in Example 5-8, the reversible compressors require 122 kJ/kg (A)
and 108 kJ/kg (B). The actual work through each compressor is determined using the
flow rates computed in step 2 and the 80% efficiency:
1122 kgkJ 2 1237.9 min2
kg
·
WS,A 5
0.8
1108 kg2 1237.9 min2
The total work required to make each
kilogram of liquid
nitrogen is 684 kJ in
the Linde process,
compared to 991 kJ in
Example 5-8.
kJ
min
5 32, 117
kJ
min
(5.118)
kg
kJ
·
WS,B 5
5 36, 280
0.8
The total work required, per kilogram of liquid nitrogen produced, is thus:
kJ
kJ
·
WS, tot 36,280 min 1 32,117 min
kJ
5 684
· 5
kg
m
kg
8
100
min
(5.119)
Chapter 5 has examined application of thermodynamics principles to complete processes, but the examples presented were again predominantly reliant on the availability of comprehensive data (e.g., Figure 2-3, Appendix A). Chapter 6 considers
what to do when such data isn't available.
5.5 s u M M a r y O F c h a P t e r F i v e
A heat engine converts heat into work, most commonly shaft work.
The Rankine cycle is commonly used in designing heat engines.
The efficiency of a heat engine is the net work produced divided by the total heat
added.
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231
C H A P T E R 5 Thermodynamic Processes and Cycles
A refrigeration process transfers heat from a low temperature location to a
high-temperature location.
The vapor-compression cycle is commonly used in designing refrigerators.
The coefficient of performance of a refrigeration process is the heat removed from
the low-temperature location divided by the work required to run the process.
Both heat engines and refrigerators are typically modeled as operating between
a high-temperature reservoir and a low-temperature reservoir.
The efficiency of a Carnot heat engine, and the coefficient of performance of a
Carnot refrigerator, are dependent only upon the temperatures of the high-and
low-temperature reservoirs. These values are for reversible processes, and represent maximums that cannot be exceeded by real processes.
The Linde liquefaction process can be used to condense low-boiling gases into
the liquid phase.
Phase changes, and the energy conversion associated with phase changes, play
an integral role in the Rankine cycle, the vapor-compression cycle, and the Linde
process. Consequently, we cannot design these processes effectively unless we
know, or are able to predict with a model, the temperatures and pressures at
which phase changes occur.
5.6 e x e r c i s e s
5-1.
The table below contains specifications for five different steady-state Rankine heat engines, A–E. Fill
in all the missing data. NOTE: All numbers in the
table are given as ABSOLUTE VALUES; determine whether they are positive or negative based
upon what you know about the Rankine cycle.
cycle
·
Qh (kj/
min)
·
Ws,turbine
(kj/min)
A
10,000
3000
B
5000
C
D
·
Ws,pump
(kj/min)
The table below contains specifications for six different steady-state vapor-compression refrigerators,
A–F. Fill in the missing data. NOTE: All numbers
are given as ABSOLUTE VALUES. Determine
whether they are positive or negative based upon
what you know about the vapor-compression cycle.
10
500
1000
E
5-2.
·
Qc
(kj/min)
5-3.
300
725
1500
6000
5
0.17
10
0.22
10
Ĥ Pump exit
(kj/kg)
A
3000
2650
400
403
B
2750
150
155
C
1900
1550
250
2750
2330
3000
F
1900
2.6
398
100
10,000
F
750
efficiency
of cycle
c.O.P.
800
2000
E
Ĥ condenser
exit (kj/kg)
·
Qc (kj/
min)
1200
400
C
Ĥ turbine exit
(kj/kg)
3600
1500
D
Ĥ boiler exit
(kj/kg)
E
A
·
Ws, compressor
(kj/min)
B
The table below contains specifications for six different steady-state Rankine heat engines, A–F. Fill
in all of the missing data.
D
cycle
·
Qh (kj/min)
3.3
0.8
375
Flow rate
of working
Fluid (kg/s)
net power
produced
(kw)
15
0.21
0.20
1000
5
0.27
10
500
0.18
20
400
0.22
4100
20,000
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232 Fundamentals of Chemical Engineering Thermodynamics
The table below contains specifications for six different steady-state vapor-compression refrigerators,
A–F. Fill in all of the missing data.
5-4.
Ĥ boiler exit
(kj/kg)
Ĥ compressor
exit (kj/kg)
Ĥ condenser
exit (kj/kg)
100
A
1000
1350
B
1500
1750
C
250
D
coefficient of
performance
1000
2200
3.5
500
10
0.9
1500
F
15,000
4
1900
750
The table on the right contains specifications for four
different liquefaction processes, A–D, all of which
are designed using the basic flow sheet shown in
Figure E5-5. Fill in the missing data.
a
n· 1smolymind
n· smolymind
8000
50,000
b
c
d
100
100
2
250
30
4
5
100
100
200
n· 3smolymind
n· smolymind
6
rate of heat
removal from
refrigerated
space (kj/min)
40
1000
1200
E
5-5.
Ĥ valve exit
(kj/kg)
Flow rate of
refrigerant
(kg/min)
n· 5smolymind
n· smolymind
300
H1sJymold
3000
2000
H 2sJymold
1500
1800
H 3sJymold
1300
H 4sJymold
50
100
6
Compressors and
heat exchangers
1
Countercurrent
heat exchanger
2
WS,tot
Qtot
Flash
3
4
Figure e5‑5
2500
1000
750
700
250
H 5sJymold
1300
900
H 6sJymold
·
WS, tot (kJ/min)
·
Qtot (kJ/min)
1600
1400
1500
35
30
55
2650
260
5.7 p r O b L e M s
5-6.
This problem examines the Rankine heat engine
introduced in Figure 5-5. Saturated steam at T =
250°C enters the turbine and the condenser operates at T = 40°C.
A. Assuming the turbine is reversible, give your best
estimate of the efficiency of the cycle, and indicate the quality of the stream leaving the turbine.
B. Assuming the turbine has an efficiency of 75%,
give your best estimate of the efficiency of the
cycle, and indicate the quality of the stream leaving the turbine.
C. Find the flow rate of circulating water needed
to produce a net power of 1 MW from the cycle
with the turbine efficiency of 75%.
5-7.
A refrigerator runs on the vapor-compression cycle.
The boiler operates at T = 265 K and the condenser
operates at 305 K. The compressor has an efficiency
of 85%. Thermodynamic data for two different
refrigerants is located in Appendix F.
A. What is the maximum attainable coefficient of
performance for Freon® 22?
B. What is the maximum attainable coefficient of
performance for refrigerant HFC-134a?
C. Besides coefficient of performance, two
considerations in choosing a refrigerant
are price and safety. Research Freon® 22 and
HFC-134a and comment on their suitability as
refrigerants.
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C H A P T E R 5 Thermodynamic Processes and Cycles
5-8.
5-9.
The engine on a steam ship runs on the Rankine
cycle. The steam leaves the boiler at 20 bar and
350°C. The turbine has an efficiency of 75% and
an outlet pressure of 1 bar. The pressure changes
in the boiler and condenser can be considered
negligible, and the liquid leaving the condenser is
saturated.
A. Determine the operating temperature of the
condenser, and compute the efficiency of a Carnot cycle operating between the boiler and condenser temperatures.
B. Determine the actual efficiency of this Rankine
cycle and compare it to the Carnot efficiency.
C. When the Titanic was sinking, the Carpathia received the S.O.S. and immediately set course to
attempt a rescue. The captain of the Carpathia
ordered the hot water turned off in the passengers’ cabins (Lord, 1955). What effect do you expect this action had?
A schematic of a variation on the Rankine cycle is
shown in Figure P5-9—not for steam but for an organic fluid. This process has been called the “organic
Rankine cycle”:
A. Do some research and determine the major advantage of using a Rankine cycle with an organic
as a working fluid as opposed to water.
B. There are five unit operations in the process in
Figure P5-9. Describe what is happening in each
of those steps (for the organic working fluid).
C. There is a valve between line 4 ➝ 5. Why do you
think that line exists and why do you think that
valve is there?
Generator
5
4
THERMAL OIL
3
8
Regenerator
Boiler
9
6
2
WATER
1
Condenser
Pump
Figure P5‑9
5-10. A refrigerator runs on the vapor compression cycle,
using HFC-134a as a refrigerant. The boiler operates
233
at 20°F. The effluent from the condenser is 10°F
above ambient temperature. The compressor has an
efficiency of 80%.
Find each of the following:
the temperature and pressure of the gas leaving
the compressor
the fraction of vapor in the stream leaving the
expansion valve
the coefficient of performance
the mass flow rate of refrigerant needed to attain
10 kJ/s of cooling
A. The ambient temperature is 70°F.
B. The ambient temperature is 110°F.
5-11. A heat engine operates on the Rankine cycle, with
saturated steam at T = 350°C leaving the boiler, a
condenser operating at T = 100°C, and a turbine
efficiency of 80%.
A. Find the liquid fraction leaving the turbine.
B. Find the overall efficiency of the heat engine.
C. A superheater is inserted into the cycle after the
boiler, which increases the steam temperature to
450°C without changing its pressure. All other
specifications remain the same. Find the liquid
fraction leaving the turbine and the overall efficiency of the cycle, and compare to the answers
from A and B.
D. A heat engine operates with the same boiler,
pump, and condenser specifications used in parts
A and B (no superheater). Instead of a single
turbine, there are two turbines, each with 80%
efficiency. The steam leaving the first turbine has
P = 3 bar, is sent to a heat exchanger in which
its temperature is increased to 200°C, and then
continues to the second turbine. Find the liquid
fractions in both turbine effluent streams and the
overall efficiency of the heat engine, and compare to the answers in A, B, and C.
5-12. Water is the most common working fluid in a Rankine heat engine, but there is no fundamental reason why a Rankine heat engine couldn’t be designed
with other working fluids. Suppose a heat engine is
to operate with the following specifications:
Quality (q) of fluid exiting turbine must be at
least 0.9.
The fluid leaving the boiler is vapor at T=100°C.
Its pressure can be specified as needed to meet
the constraint that q > 0.9.
The fluid leaving the condenser is saturated
liquid at T = 40°C
Turbine efficiency is = 0.8
Find the state of the fluid exiting the turbine, the
net work produced by the cycle per kilogram of
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234 Fundamentals of Chemical Engineering Thermodynamics
operating fluid, and the efficiency of the cycle for
each of the following operating fluids.
A. Water
B. Ammonia (Use Figure 6-2 for data)
5-13. A steady-state liquefaction process generates
100 lbm/min of saturated liquid methane at
P = 10 psia. Fresh methane enters the process at
T = 75°F and P = 10 psia, and is mixed with the
recycled methane to form the “methane feed”
stream. This enters a heat exchanger (HX1) that
cools the methane feed to T = –100°F and P = 10
psia. The process has two compressors (both =
0.75: the first compresses the methane to P = 100
psia and the second compresses the methane to
P = 1000 psia). Each compressor is followed by
a heat exchanger (HX2 and HX3) that cools the
methane to T = –100°F without changing the
pressure. Next, the supercritical methane enters
a counter-current heat exchanger (HX4) in which
the methane vapor from the flash chamber is
used as the coolant. The coolant leaves HX4 at
T = –110°F and is recycled into the “methane
feed” stream. The supercritical methane leaves
HX4 and enters a flash chamber, where its pressure
is reduced to P = 10 psia.
A. Determine the flow rate of the supercritical
methane entering the flash chamber.
B. Determine W for each of the two compressors.
C. Determine Q for each of the four heat exchangers.
5-14. The boiling point of a compound at P = 0.1 MPa is
150 K. The Linde liquefaction process will be used to
produce saturated liquid at P = 0.1 MPa, which has
a specific enthalpy of 20 kJ/kg. The Table P5-14 in
the right-hand column contains some physical properties of the compound. The steady-state process
works as follows.
Step 1. 50 kg/minute enters the process at T = 250
K and P = 0.1 MPa.
Step 2. The feed enters a series of compressors and
heat exchangers, and it leaves the last heat
exchanger at T = 200 K and P = 10 MPa.
The TOTAL work added by the compressors is 300 kilojoules per kilogram of feed.
Step 3. The stream leaving step 2 is cooled to T =
175 K in a heat exchanger.
Step 4. The stream leaving step 3 enters a flash
chamber where it expands to 0.1 MPA and
some of it condenses.
Step 5. The vapor from the flash chamber is used
as the coolant for the heat exchanger in
step 3.
Step 6. The vapor stream (at P = 0.1 MPA) from
step 5 is NOT recycled; it exits the process
as a by-product.
A. Find the flow rate of liquid product leaving the
flash chamber.
B. Find the specific enthalpy (kJ/kg) of the vapor
by-product described in step 6.
C. Find the total heat removed by the heat exchangers during step 2, in kJ/min.
D. What is the heat capacity of the compound, in
kJ/kg ? K, at ideal gas conditions?
Specific enthalpy in vapor or supercritical phase at various
temperatures and pressures, in kJ/kg.
150 K
175 K
200 K
225 K
250 K
275 K
0.1 MPa
200
215
230
245
260
275
1 MPa
192
202
215
229
241
258
10 MPa
184
190
200
215
228
241
5-15. A steady-state heat engine operates on the Rankine
cycle.
The steam entering the turbine is 1 kg/s of steam
at P = 3.5 MPa and T = 350°C.
The ACTUAL stream exiting the turbine is a mixture of 95% vapor and 5% liquid at P = 50 kPa.
The stream entering the pump is saturated liquid
at P = 50 kPa.
A. Determine the efficiency of the turbine.
B. Estimate the work required by the pump.
C. Estimate the overall efficiency of the heat engine.
D. What is the efficiency of a Carnot heat engine that
operates between a high temperature of 350°C
and a low temperature that is the same as the
temperature of the condenser in this problem?
5-16. A refrigeration process operates using the vaporcompression cycle, using a proprietary refrigerant
that the inventor claims is better than R-134a. At low
pressures, the refrigerant can be assumed to act as an
ideal gas with constant CV = 14R and reportedly has
a molecular weight of 400 g/mol. The steady-state
refrigeration cycle reportedly works as follows:
A boiler produces saturated vapor at P = 0.02 MPa
and T = 5°C, which has H = 50 kJ/mol.
The vapor is compressed to P = 0.06 MPa and
T = 50°C.
The vapor is then condensed to saturated liquid
at P = 0.06 MPa, which has H = 20 kJ/mol.
The liquid undergoes an isenthalpic expansion to
P = 0.02 MPa, and enters the boiler.
A. What is the efficiency of the compressor?
B. If we wished to scale the process described above
so that it provides 3000 kJ/min of cooling, what is
the required flow rate of refrigerant in kg/min?
C. What is the coefficient of performance of the
refrigeration cycle?
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C H A P T E R 5 Thermodynamic Processes and Cycles
D. Design a refrigeration cycle using R-134a as
the refrigerant, in which the rate of cooling
is 3000 kJ/min. In designing this process, you
should make all specifications and design decisions in such a way as to allow a meaningful
comparison between R-134a and the proprietary refrigerant.
E. Based on the outcomes of questions A through
D, comment on the inventor’s claim that the proprietary refrigerant is better than R-134a.
5-17. A steady-state Rankine cycle currently in service
operates as follows:
Steam leaves the boiler at P = 8 bar and T = 250°C.
Steam leaves the turbine as saturated vapor at
P = 0.3 bar.
Water leaves the condenser as saturated liquid at
P = 0.3 bar.
Water leaving the condenser is pumped up to 8
bar and returned to the boiler.
The power output FROM THE TURBINE
is 1 MW.
Your company has the opportunity to upgrade the
turbine to one that is 85% efficient, at a cost of
$1.2 million. This includes all costs associated with
the replacement—the new equipment, installation,
instru-mentation, etc. If the upgrade is done it will
work this way:
The flow rate, temperature, and pressure of the
steam entering the turbine will all be unchanged,
but the turbine is expected to produce more
work.
The pressure leaving the turbine will still be
P = 0.3 bar, but won’t necessarily be saturated
vapor, since the turbine is now removing more
energy as work.
The QC in the condenser will be adjusted such
that the water leaving the turbine is saturated
liquid, allowing the pump and boiler to operate
EXACTLY the same in the upgraded cycle as
they do in the current cycle.
The current turbine produces 1 MW of power, 24
hours a day, for 350 days per year. The company
values any “extra” work produced beyond this at
$20/GJ.
A. Determine the flow rate at which water/steam
circulates through the process.
B. Determine the efficiency of the turbine in the
CURRENT cycle.
C. Determine the overall efficiency of the CURRENT cycle.
D. Determine the power produced by the turbine in
the UPGRADED cycle.
235
E. Determine the overall efficiency of the UPGRADED cycle.
F. How long will the new turbine have to operate
in order to pay for the $1.2 million cost of the
upgrade?
5-18. You are designing a steady-state liquefaction process that will manufacture liquid methane. Part A of
this problem will focus on two unit operations: the
flash separation step itself, and the counter-current
heat exchanger in which the vapor from the flash
chamber is used to cool the feed entering the flash
chamber. In answering parts B and C, however, consider the entire Linde process, not just the two unit
operations for which you performed calculations.
Methane enters the counter-current heat exchanger as supercritical vapor at P = 1000 psia and
T = –100°F. It is cooled and enters the flash chamber,
where the pressure is reduced to P = 10 psia. The
flow rate of liquid methane product is 100 lbm/min.
The methane vapor from the flash is sent to the
counter-current heat exchanger. The design parameter
that is under your control is the temperature of this
methane vapor stream when it leaves the countercurrent heat exchanger—it can be –130, –120, or –110°F.
A. Find the flow rate of supercritical methane entering the counter current heat exchanger for each
of the three possible systems.
B. Discuss the factors that you would expect to
affect the cost OF THE EQUIPMENT ITSELF
for the Linde process, and what the results of
part A suggest about these costs.
C. Discuss the factors that you would expect to affect
the cost of OPERATION of the process, and what
the results of part A suggest about these costs.
5-19. You are designing a refrigeration cycle, and have
the option of using either a compressor with an
efficiency of 70% or a compressor that is more expensive by $5000 but has an efficiency of 80%. The
following specifications are valid regardless of which
compressor is used:
The refrigerant is HFC-134a.
The liquid leaving the condenser is saturated liquid at T = 50°C.
The boiler operates at T = 0°C.
The vapor leaving the boiler is saturated vapor.
The cycle must have QC = 1000 kJ/min in the
boiler.
A. Determine the flow rate of refrigerant. Is it the
same or different in the two cycles?
B. Determine the compressor work for each of the
two possible compressors.
C. Determine the coefficient of performance for the
cycle with each of the two possible compressors.
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236 Fundamentals of Chemical Engineering Thermodynamics
D. The compressor runs on electricity, which is available for $0.10/kWhr. Assuming the refrigeration
system runs constantly, how long would the system have to run in order for the higher-efficiency
compressor to be cost effective?
Operating
temp (°c)
exchanger
5-20. You are designing a Rankine heat engine which
must produce 10,000 kJ/min of NET shaft work. The
heat source is at T 5 200°C and the low-temperature
reservoir at T 5 25°C. The turbine efficiency is 75%,
and there are no restrictions on the temperature,
pressure, or quality of the water leaving the turbine.
Your job is to design the most cost-effective Rankine engine possible. The boiler can be designed
to operate at T 5 190°C, T 5 185°C, or T 5 180°C,
and the condenser can be designed to operate at
T 5 35°C or T 5 40°C. Costs of these heat
exchangers can be determined from the formulas in
the table on the right. Notice that the cost of the heat
exchanger goes up as the heat duty goes up, and
goes down as the ΔT between the heat reservoir and
the fluid increases.
Formula
Boiler
180
·
C = 10000 + Q
Boiler
185
Boiler
190
Condenser
35
Condenser
40
·
C = 12000 + 1.5Q
·
C = 15000 + 2Q
·
C = 10000 + 1.5Q
·
C = 7000 + Q
·
In all formulas, C represents the cost in dollars and Q
represents the absolute value of the heat transferred in that
exchanger in kJ/min.
The cost of the heat added to the boiler is $15/GJ.
The heat removed in the condenser has neither cost
nor value. Assume that the heat engine will be operating 24 hours per day, 350 days per year.
A. For each of the six possible Rankine cycles, de· ·
termine QH, QC, the cost of the two heat exchangers, and the yearly cost of the heat.
B. Recommend which variation of the Rankine
heat engine should be used if it’s expected to be
in service for 5 years.
5.8 g LO s s a r y O F s yM b O L s
area
A
C.O.P. coefficient of performance
CP
constant pressure heat
capacity
CP*
constant pressure heat capacity for ideal gas
constant volume heat
capacity
CV
constant volume heat capacity for ideal gas
CV*
M
·
m
n·
mass of system
Ŝ
specific entropy
mass flow rate
molar flow rate
S
·
Sgen
rate of entropy generation
P
pressure
T
temperature
Q
heat
TC
QC
heat exchanged with a lowtemperature heat reservoir
temperature of a lowtemperature heat reservoir
TH
QH
heat exchanged with a hightemperature heat reservoir
temperature of a hightemperature heat reservoir
V
volume
rate of heat addition
V̂
specific volume
rate of heat exchange with
low temperature reservoir
W
·
W
work
WS
shaft work
ΔĤvap
enthalpy of vaporization
efficiency
H.E
overall efficiency of a heat
engine
·
Q
·
QC
carnot
overall efficiency of a completely reversible heat engine
·
QH
H
enthalpy
Ĥ
specific enthalpy
H
molar enthalpy
rate of heat exchange with
low temperature reservoir
q
quality, or vapor fraction
R
gas constant
S
entropy
molar entropy
power
5.9 r e F e r e n c e s
Lord, W. A Night to Remember. Henry Holt and Company,
1955.
Richard C. Bailie, Wallace B. Whiting, Joseph A. Shaeiwitz,
and Richard Turton, Analysis, Synthesis and Design of
Chemical Processes, 3rd ed. Prentice-Hall, 2009.
Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
6
Thermodynamic Models of
Real, Pure Compounds
Learning Objectives
This chapter is intended to help you learn to:
Write total derivative expressions that relate unknown variables to known variables
Manipulate partial derivatives using mathematical tools such as the triple product
rule and expansion rule
Apply the principles of calculus to express partial derivatives in terms of measurable
properties
Combine the mathematical techniques mentioned above with equations of state to
solve problems in the absence of thermodynamic property data
Use residual properties to solve problems involving real gases, when heat capacity is
only known in the ideal gas state
T
he previous five chapters gave many examples of engineering problems that can be
solved with energy and entropy balances. Solving these problems required knowledge of properties such as U, H, and S. In many cases we made use of experimental
values for these properties, but we have noted several times (starting in Chapter 2) that we
can’t assume that such data will always be available. The ideal gas model has been used
extensively in Chapters 3–5, but its use is limited to applications at very specific conditions.
In modeling liquids and solids, we have assumed volume was constant with respect to both
temperature and pressure, another simplifying approximation that is not always reasonable.
In this chapter, we will introduce mathematical modeling strategies that are applicable
to pure compounds at a broad range of conditions.
6.1 MOtivatiOnaL exaMPLe: joule-thomson expansion
We begin by posing a problem that should at this point appear comparatively routine:
expansion in a throttling valve. This kind of example was first examined in Section
3.6.1, and it was shown that if the valve is adiabatic and operating at steady state, the
energy balance simplified to:
Ĥin 5 Ĥout
or
H in 5 H out
(6.1)
Thus, the process is isenthalpic; the gas or liquid entering the valve experiences
a sudden decrease in pressure but the enthalpy is unchanged. Isenthalpic expansion, which is also known as Joule-Thomson expansion, was an essential step in the
refrigeration and liquefaction cycles examined in Chapter 5. In refrigeration, the
Section 5.3 (refrigeration) and Section 5.4
(liquefaction) both
illustrate processes in
which Joule-Thomson
expansion plays a
pivotal role.
237
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238 Fundamentals of Chemical Engineering Thermodynamics
expansion led to a phase change; a high-pressure saturated liquid entered the valve,
and a portion of the liquid evaporated due to the pressure change. Because no work
or heat was added to provide the enthalpy of vaporization, the energy came from the
liquid itself, resulting in a large temperature drop. In the Linde liquefaction process,
the feed to the Joule-Thomson expansion step was a supercritical fluid rather than a
saturated liquid, but Examples 5-8 and 5-9 again showed a liquid–vapor mixture was
produced and a significant temperature change occurred.
Example 6-1 illustrates the expansion of gaseous ammonia, and shows how
Joule-Thomson expansion can have a significant effect on the temperature of a
vapor, even in the absence of a phase change.
examPle 6‑1
jOuLe-thOMsOn expansiOn OF aMMOnia
Ammonia vapor enters an adiabatic, steady-state throttling valve at T = 30°C and P =
10 bar, illustrated in Figure 6-1. If it leaves the throttling valve at P = 1 bar, what is the
exiting temperature?
Ammonia
P 5 10 bar
T 5 308C
P 5 1 bar
T5?
Figure 6‑1 Joule-Thomson expansion
examined in Example 6-1.
60
708C
0
5.
00
5
s5
5.
25
s5
00
6.
208C
s5
108C
6.
s5
08C
2108C
s51.00 kJ/kgK
2 20
2 30
1.00
0.90
0.80
0.70
0.60
0.50
75
5.
s5
308C
3.00
2.00
5.
25
50
6.
s5
75
6.
s5
2208C
00
s5
2308C
2408C
2 40
7.
25
7.
s5
50
7.
s5
5
7.7
s5
K
/kg
kJ
240 220 0 20 40 60 80 100 120 140 160 180 200 220
8C 8C 8C 8C 8C 8C 8C 8C 8C 8C 8C 8C 8C 8C
0
100
200
300
400
500
600
700
800
900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Enthalpy (kJ/kg)
Figure 6‑2 Pressure-enthalpy diagram for ammonia.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Based on http://www.softpedia.com/get/Multimedia/Graphic/Graphic-Others/CoolPack.shtml
s52.00
408C
210
4.00
50
s5
608C
508C
10
10.00
9.00
8.00
7.00
6.00
5.00
s5
4.7
s5 4
1208C
1108C
1008C
908C
808C
20
30
40
20.00
.50
.25
s5 4
.00
.75
s5 3
.50
s53
.25
s53
s5 4
130
120
s53
80
70
1308C
50
30.00
Pressure (bar)
.00
90
100
100.00
90.00
80.00
70.00
60.00
50.00
40.00
110
200.00
C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
239
SOLUTION:
Step 1 Apply energy balance
If we define the valve as the system, then we have a steady-state, adiabatic throttling
process; the exact situation described by Equation 6.1
Ĥin 5 Ĥout
Step 2 Look up data
Figure 6-2 shows that the specific enthalpy of ammonia at P = 10 bar and T = 30°C
is 1500 kJ/kg; this is Ĥin. The energy balance tells us that the exiting stream has
Ĥout = 1500 kJ/kg, and at P = 1 bar, this specific enthalpy falls between the T = 0°C and
T = 10°C isotherms in Figure 6-2.
Tout , 3°C.
Example 6-1 illustrates the prediction of the temperature change that occurs when
a gas expands. U.S. Patent #5,522,870 (Ben-Zion, 1994) gives an example of an
application for this phenomenon. The patent describes a device that can either cool
or heat a surface very rapidly, allowing one to produce large temperature changes
almost instantly and with precise timing. The patent cites cryosurgery, surface
curing, and sealing as applications. Thus, we can recognize Example 6-1 as a practical and important kind of problem, but we wouldn’t have been able to solve the
problem without Figure 6-2. This chapter, in a nutshell, addresses the question
“what do you do when comprehensive data (such as Figure 6-2 in Example 6-1) is
not available?”
The chapter explores the construction of mathematical models that accurately
describe the inter-relationship between physical and thermodynamic properties, including U, H, S, P, V, and T. Such models have two major purposes.
1.
2.
“Cryosurgery” is the
use of extreme cold to
destroy malignant or
abnormal tissue. The
ability to change the
temperature of instruments quickly is helpful in localizing the
effect of the treatment
to the targeted tissue.
Models allow us to solve problems in the absence of data. It is not realistic to
assume that the kind of data given in Figure 6-2 will be available for all compounds at all temperatures and pressures of interest; there’s a need for accurate
strategies for estimating quantities that have never been measured.
The principles and techniques presented in Chapters 6–8 not only allow
construction of models that describe the properties of pure compounds, but
also lay the foundation for modeling of mixtures, which we cover beginning in
Chapter 9.
Models help us to gain deeper insights into the behavior of real chemicals, and
the cause-effect relationships that occur between parameters in chemical processes. Patent #5,522,870, mentioned above, describes a device that can rapidly
cool a surgical probe to cryogenic temperatures, and just as rapidly return the
device to room temperature. Interestingly, the heating and cooling are both
achieved by Joule-Thomson expansions. We’ve now seen several examples
(Examples 6-1, 5-6, and 5-4) in which Joule-Thomson expansion produced cooling; how can the same process also produce heating? Observation and data can
tell us “most gases cool when they expand, but some gases actually increase
in temperature,” but the mathematical models developed in this chapter (see
particularly Example 6-3) will give us a quantitative basis for rationalizing these
apparently contradictory observations.
Example 6-1 essentially posed the question: If ammonia vapor is expanded isenthalpically from P = 10 bar to P = 1 bar, what is the change in temperature? The next
section begins to explore how to answer this kind of question without comprehensive data relating H to T and P.
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240 Fundamentals of Chemical Engineering Thermodynamics
6.1.1 the total derivative
We begin by noting a rule of partial differential calculus:
If a variable X is a function of Y and Z, then changes in X can be related to changes in
Y and Z through a total derivative:
FOOd FOr
thOught
6-1
Why is X written as
a function of two
variables, Y and Z?
Suppose we related X
to three variables Y,
Z, and W:
dX 5
dY
1−X
−Y 2
W, Z
1
1
1−X
−Z 2
dZ
dX 5
Z
Y, Z
Isn’t this just as valid
as Equation 6.2?
Thus, if we let X, Y, and Z represent, for example, U, T, and S, Equation 6.2 would
become
dU 5
1 −T 2
−U
S
Here it is stressed that
X, Y, and Z in Equation 6.2 can represent
any intensive variable
(e.g, H). Why not an
extensive variable?
Equations 6.2 through
6.5 are the first of dozens of equations that
will be presented in
this chapter. Attempting to solve problems
by drawing from this
maze of equations in a
haphazard or arbitrary
way will not be productive. We will use
a simple question to
guide ourselves: how
can I relate what I
want to know to what
I do know?
dT 1
1 −S 2 dS
−U
(6.3)
T
Or if we let X, Y, and Z represent molar enthalpy, temperature, and pressure, Equation 6.2 becomes
dH 5
1 −T 2 dT 1 1 −P 2 dP
−H
−H
P
FOOd FOr
thOught
6-2
(6.2)
Y
We can write such a total derivative expression for any three intensive state properties
X, Y, and Z.
W, Y
−X
1−W
2 dW
1−X−Y2 dY 1 1−Z−X2 dZ
(6.4)
T
Clearly, we could continue in this vein for some time, writing dozens more total derivative expressions. The question to ask ourselves is, which expression is the best starting
point for the problem we are trying to solve? In Example 6-1, the goal was to find the
change in temperature (T2 2 T1) resulting from a Joule-Thomson expansion. Consequently, if we wished to solve a similar problem using a model instead of data, we
would do well to start with an equation that models changes in temperature (dT ):
dT 5
−T
dP
1−H
2 dH 1 1−T
−P 2
P
(6.5)
H
Equations 6.3 through 6.5 are all correct, and we can imagine many more total
derivative equations that could be written, relating various state properties to each
other. However, in the case of Joule-Thomson expansion, Equation 6.3, while correct, is not particularly useful, since no information is given or asked about either
internal energy or entropy. By contrast, Equation 6.5 is readily applicable to a JouleThomson expansion.
The goal of Example 6-1 was to calculate the change in temperature of a vapor
resulting from a process. Since “dT ” represents precisely what we are trying to
find, it makes sense to start by writing a total derivative expression for dT.
In Example 6-1, the two properties we knew the most about were pressure (initial and final P are given) and enthalpy (we knew from the energy balance that
the process was isenthalpic). Consequently, it is quite natural to let Y and Z in
Equation 6.2 represent pressure and molar enthalpy; the resulting Equation 6.5
relates what we want to know to what we do know.
Example 6-2 demonstrates the application of a total derivative to solving a problem.
It examines a Joule-Thomson process identical to that shown in Figure 6-1 and Example 6-1, except that instead of ammonia, the substance flowing through the valve
is an ideal gas.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
jOuLe-thOMsOn expansiOn OF an ideaL gas
241
examPle 6‑2
An ideal gas with a constant CP* = (7/2)R enters a steady-state throttling valve at P =
10 bar and T = 30°C. If it leaves the valve at P = 1 bar, what is the exiting temperature?
P 5 1 bar
T5?
P 5 10 bar
T 5 308C
Figure 6‑3 Joule-Thomson expansion
of an ideal gas, examined in Example 6-2.
SOLUTION:
Step 1 Define a system and apply energy balance
Defining the system as the throttling valve, the energy balance, as derived in Section 3.6.1, is:
H in 5 H out
(6.6)
Notice that we do not have numerical values for H in and H out, we simply know they are
equal to each other, which proves to be sufficient to solve the problem.
Step 2 Write a total derivative
We wish to find the unknown outlet temperature of the gas. The two intensive state properties we know the most about are pressure (P) and molar enthalpy (H ). This is the exact
situation for which Equation 6.5 was proposed:
dT 5
For this problem, it
makes no real difference whether enthalpy
is expressed on a
molar or mass basis.
However, equations
of state are normally
written on a molar
basis, and we will use
equations of state extensively throughout
this chapter, so using
molar properties is
more convenient in
general.
1−H2 dH 1 1−P2 dP
−T
−T
H
P
This equation is not specific to our valve; T, P, and H are all state properties and this
equation can be applied to any process involving a pure compound. Here, if we define the
state of the entering gas as “state 1” (P1 = 10 bar, T1 = 30°C) and the state of the exiting
gas as “state 2,” (P2 = 1 bar), we know from the energy balance that H 1 = H 2. Equation 6.5 can be solved by integrating from state 1 to state 2.
# dT 5 # 1−H2 dH 1 # 1−P2 dP
T2
H2
T1
H1
−T
P2
P1
P
−T
H
(6.7)
Step 3 Simplify total derivative
The first term on the right-hand side (the dH term) is zero. Mathematically, this is because H 1=H 2; if the limits of integration are identical, the integral is zero, no matter what
(∂T/∂H )P is. Physically, this can be understood as follows: (∂T/∂H )P quantifies the effect
of a change in enthalpy on temperature, but in this case the enthalpy is not changing so
the value of (∂T/∂H )P doesn’t matter. Equation 6.7 simplifies to
# dT 5 # 1 2
T2
P2
T1
P1
−T
dP
−P H
(6.8)
In order to integrate (∂T/∂P)H with respect to dP, we have to express (∂T/∂P)H as a
function of P.
Step 4 Relate Joule-Thomson coefficient to measurable properties
Using a rule of partial differential calculus called the triple product rule, which is
The manipulations in
Step 4 are likely hard
to follow because they
are unfamiliar. Sections 6.2.1 through
6.2.4 provide a detailed discussion of the
mathematics used in
this step.
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242 Fundamentals of Chemical Engineering Thermodynamics
explained in Section 6.2.1, we can express (∂T/∂P)H as
1 −P 2
52
2
1−T
−H
−P
1 −T 2
−H
T
(6.9)
H
P
By definition (∂H/∂T)P is equal to CP, so
1 −P 2
−H
1−P2 5 2 C
−T
H
T
(6.10)
P
Another rule of partial differential calculus called the expansion rule is presented in Section 6.2.3. We can use the expansion rule to prove:
1 −P 2 5 V 1 T 1−P2
−H
−S
T
(6.11)
T
But an expression known as one of Maxwell’s relations states that
1 −T 2 5 21−P2
−V
The differential expression (∂V/∂T )P has
a particular physical
significance in thermodynamics that is
discussed in Section
6.2.5; the coefficient of
thermal expansion of
a substance is defined
as (1/V ) (∂V/∂T )P.
−S
P
Substituting Equations 6.11 and 6.12 into Equation 6.10 reveals that
1−P2 5 2
−T
V2T
1 −T 2
−V
P
(6.13)
CP
H
Consequently, Equation 6.8 can be re-written as
# dT 5 # 2
T2
T1
R is always a constant.
We are evaluating the
derivative with respect
to T at constant P, so
we treat P as a constant in evaluating this
partial differential.
Thus, the right hand
side of Equation 6.15
is essentially a constant (R/P) times T,
and the derivative of T
with respect to T is 1.
(6.12)
T
P2
V2T
1 −T 2
CP
P1
−V
P
dP
(6.14)
These manipulations are discussed in detail in Section 6.2. For now, consider this: why
is Equation 6.14 “better” than Equation 6.8? At first glance they both appear to have
the same problem; neither can be integrated with respect to dP because the integrand
is not expressed as a mathematical function of pressure. However, the expression
−V
V2T
can be related to pressure through an equation of state. Here, the gas can
−T P
be modeled using the ideal gas law.
1 2
Step 5 Evaluate integral for ideal gas law
According to the ideal gas law, molar volume can be expressed as
V5
RT
P
(6.15)
Next, we can evaluate (∂V/∂T )P by taking the derivative of both sides of Equation 6.15
with respect to T at constant P:
1 −T 2 5 RP
−V
(6.16)
P
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
Plugging Equations 6.15 and 6.16 into Equation 6.14 gives
1 2
pitFaLL
preventiOn
−V
RT RT
V2T
2
T2
P2
P2
−T P
P
P
dT 5
2
dP 5
2
dP
CP
CP
P1
P1
T1
#
#
# dT 5 #
T2
P2 51 bar
T1
P1 510 bar
#
(6.17)
0 dP
(6.18)
T2 2 T1 5 0sP2 2 P1d
T2 5 T1
Equation 6.18 shows that the temperature of an ideal gas is unchanged in Joule-Thomson
expansion.
Example 6-2 demonstrated that modeling Joule-Thomson expansion requires evaluating the partial derivative (∂T/∂P)H. Consequently this partial derivative is called
the Joule-Thomson coefficient, JT :
JT 5
52
1−T
−P 2
V2T
1 −T 2
You probably remember from calculus
that “the integral of
0 is a constant.” That
memory sometimes
causes confusion in
evaluating an expression like 0 dP. The
key is the distinction
between definite and
indefinite integrals.
When you evaluate a
definite integral, there
is no “constant of integration”; the definite
integral of 0 dP is 0.
−V
P
CP
H
243
(6.19)
The Joule-Thomson coefficient in Example 6-2 was 0, but this result was for an ideal
gas specifically. The more general equation is
V2T
dT 5 JT dP 5 2
1 −T 2
−V
P
CP
dP
(6.20)
The van der Waals
equation of state was
first introduced in
Section 2.3.4.
which is valid for any Joule-Thomson expansion. The next example uses the van der
Waals equation of state.
jOuLe-thOMsOn expansiOn OF a van der waaLs gas
A gas has CP = (7/2)R and is described by the van der Waals equation of state with a = 0
and b = 150 cm3/mol. The gas enters a steady-state throttling valve at P = 10 bar and
T = 30°C and leaves the valve at P = 1 bar. What is the exiting temperature?
SOLUTION:
Step 1 Define a system and write an energy balance
As in Example 6-2, if we define the system as the valve, the energy balance simplifies to
Equation 6.1:
examPle 6‑3
Here, the heat capacity is modeled as
constant for simplicity,
as CP is not the focus
of the example. Section 6-3 presents more
realistic methods of
accounting for heat
capacity of a gas.
H in 5 H out
Step 2 Compare and contrast with Example 6-2
Our goal is to relate changes in temperature (T ) to pressure (P) and specific enthalpy
(H), so we can apply known information regarding P and H to find the outlet temperature (T2). Steps 2 through 4 of Example 6-2 did exactly this, writing and simplifying a total
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244 Fundamentals of Chemical Engineering Thermodynamics
derivative expression that related changes in temperature (dT) to changes in pressure
(dP) and specific enthalpy (dH) (Equation 6.14):
# dT 5 # 2
T2
P2
T1
P1
V2T
1 −T 2
−V
P
CP
dP
Here, as in Example 6-2, we have an isenthalpic expansion, and Equation 6.14 is again
valid. Now, however, instead of an ideal gas, we have a gas that is described by the van
der Waals equation of state.
Step 3 Evaluate
1 −T 2 for this gas
−V
P
The van der Waals equation of state is Equation 2.53:
P5
RT
a
2
V 2 b V2
However, for this gas, a = 0, so the equation simplifies to
P5
a and b are constants
in the van der Waals
equation, so the derivative of either with
respect to T (or anything else) is 0. We will
see in Chapter 7 that
there are other equations of state that have
parameters called a
and b, but in which a
is a function of temperature, rather than a
constant.
RT
V2b
(6.21)
1 2
−V
The most straightforward way to evaluate
is to first solve the equation of state
−T P
for V:
RT
1b
P
V5
(6.22)
and then to differentiate both sides with respect to T (treating P as a constant):
1 −T 2 5 RP
−V
(6.23)
P
Step 4 Solve the integral
In order to integrate Equation 6.14 with respect to dP, we need to express the integral as
−V
in terms of P:
a function of P. So we use Equations 6.22 and 6.23 to express V and
−T P
1 2
# dT 5 # 2
T2
P2
T1
P1
V2T
1 −T 2
−V
CP
P
dP 5
12
P2
R
RT
1b2T
P
P
P1
CP
# 2
dP
(6.24)
# dT 5 # 2 C dP
FOOd FOr
thOught
6-3
Is it a problem that
temperature is expressed in Celsius on
the left-hand side of
Equation 6.25, but in
Kelvin on the righthand side?
T2
P2
b
T1
P1
P
1 Cb 2sP 2 P d
T2 2 T1 5 2
2
1
P
T2 2 T 1 5
31
2
150
cm3
mol
7
bar ? cm3
83.14
2
mol ? K
24
s1 2 10 bard 5 4.6 K
(6.25)
T2 5 34.68C
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
245
Examples 6-1 through 6-3 all show a gas experiencing an adiabatic pressure drop.
In Example 6-1, the temperature decreased.
In Example 6-2, the temperature was unchanged.
In Example 6-3, the temperature increased.
Why the differences? The reason the ammonia in Example 6-1 decreased temperature on expansion was because of attractive intermolecular forces. A decrease in
pressure means the molecules move farther apart. If the molecules are attracted to
each other, then it takes energy to move them farther apart. In effect, the molecules
are gaining microscopic potential energy, much like an object gains potential energy
when it is moved farther from the Earth, to which its attracted by gravity. Where
does this energy “come from”? With no external source of energy, it comes from the
ammonia itself- the molecules gain microscopic potential energy and lose microscopic kinetic energy, observed as a decrease in temperature.
But in Example 6-3, we set a 5 0. The van der Waals a represents the very attractive forces that the last paragraph described. If there are no attractive forces
then there is no temperature drop. But why a temperature rise? The van der Waals b
represents excluded volume, and the repulsive forces resulting when molecules that
have a finite volume are compressed close together. Thus we see the opposite phenomenon: the intermolecular forces are repulsive not attractive, so when the molecules are spread apart (lower P) we see a temperature increase.
The ideal gas in Example 6-2, which had neither repulsive nor attractive forces,
had no temperature change at all. Real gases, of course, have both attractive and
repulsive intermolecular forces. The result of Example 6-1 shows that the attractive forces are more important for ammonia at 308C and 1 bar. This is typical- most
gases at typical process conditions decrease in temperature when they expand. But
gases at high temperatures increase temperature when they expand. The inversion
temperature is the temperature at which the transition from cooling to heating takes
place; it is defined as the temperature at which (∂T/∂P)H 5 0. Inversion temperature
is different for every gas and is a function of pressure.
6.2 Mathematical Models of thermodynamic Properties
The examples in Section 6-1 illustrate the need for building mathematical models that describe thermodynamic systems and quantify the relationships between
physical properties. Our specific interest is expressing our mathematical model in
terms of the measurable properties (P, V , T ), so that a known equation of state
can be applied. This section summarizes a number of mathematical tools and techniques that are useful for building thermodynamic models. These are presented,
not in order of importance, but in the order in which they are employed in deriving
Equation 6.19.
For now when we think of “measurable properties,” we will include the heat capacity (CP or CV), and we will only consider applications in which the heat capacity
is known. Section 6-3 looks at heat capacity, and situations in which it is and is not
known, in more depth.
Equation 6.19 was
useful for solving
Example 6-2 and
Example 6-3, but the
derivation was only
described briefly. Sections 6.2.1 through
6.2.4 each discuss one
step in the derivation.
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246 Fundamentals of Chemical Engineering Thermodynamics
6.2.1 the triple Product rule
Equation 6.19 relates the Joule-Thomson coefficient to measurable properties and
was useful for solution of Examples 6-2 and 6-3:
JT 5
−T
1−P
2 52
V2T
1 −T 2
−V
P
CP
H
Notice the constant-pressure heat capacity appears on the right-hand side. Consider the
definitions of the Joule-Thomson coefficient and the constant pressure heat capacity:
This mathematical
definition of CP was
first introduced in
Section 2.3.2.
JT 5
1−T
−P 2
CP 5
1 −T 2
H
−H
P
Since they relate the same three quantities (T, P, and H) it seems quite logical that
there would be some mathematical relationship between them. The specific relationship can be understood through a useful property of partial differential calculus
called the triple product rule.
FOOd FOr
thOught
6-4
Go back to Example
6-3. Suppose a had not
been equal to 0. How
would this have made
−V
evaluating
−T P
more difficult? How
would the information in this section be
helpful for evaluating
−V
?
−T P
1 2
1 2
The triple product rule can be expressed as
1−X−Y2 1−Z−Y2 1−Z−X2 5 21
Z
X
(6.26)
Y
or equivalently as
1−X−Y2 5 −Y 21−Z
1−Z2 1−X2
(6.27)
Z
X
Y
Applying the triple product rule X 5 T, Y 5 P and Z 5 H gives
21
5
1−T
2
−H
−P
−P
1−H
2 1 −T 2
(6.28)
H
T
P
Another basic rule of partial differential calculus is
1
5
1−X
2
−Y
−Y
1−X
2
(6.29)
Z
Z
While heat capacity
cannot be measured
as simply and directly
as pressure, temperature, and volume, it
can be measured with
calorimetry. In this
section we regard CP
and CV as “measurable
properties.”
Applying Equation 6.29 and the definition of heat capacity to Equation 6.28 gives
1 −P 2
−H
52
1−T
−P 2
C
H
T
(6.30)
P
Comparing Equation 6.30 to Equation 6.19 shows that, by using the triple product rule,
we’ve now taken a significant step toward deriving Equation 6.19. To complete the pro−H
cess of relating JT to measureable properties, we now need to evaluate
.
−P T
1 2
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
247
6.2.2 Fundamental Property relationships
In this section we will derive a fundamental relationship between U and other state
properties. We will do this by considering a gas confined in a simple piston-cylinder device. Since this is a closed system, the (time-independent) energy balance simplifies to
DU 5 Q 1 W
(6.31)
In this case, an energy balance in differential form will be more convenient:
dU 5 dQ 1 dW
(6.32)
There is no mechanism for shaft work, so dW represents only work of expansion/
contraction, which is given by –P dV, as first introduced in Equation 1.21.
dU 5 dQ 2 P dV
(6.33)
Furthermore, if the process is reversible and the gas is in a homogeneous phase, we
can relate dQ to entropy through the definition of entropy (Equation 4.13):
dU 5 T dS 2 P dV
(6.34)
If we divide through by the number of moles of gas in the cylinder, we obtain
dU 5 T dS 2 P dV
Examining Equation 6.35, we note that they consist entirely of state functions:
molar internal energy, molar entropy, temperature, pressure, and molar volume.
This observation has great significance. Even though Equation 6.35 was derived
by considering a reversible process, the end result is path-independent. Equation
6.35 is thus valid for any process, whether it is reversible or irreversible. Further,
it does not matter whether the process is carried out in a piston-cylinder device.
Equation 6.35 can be used to describe the change in molar internal energy for any
pure material in any process, and this is why it is known as a “fundamental” property relationship.
We can derive an analogous fundamental property relationship for the molar
enthalpy by taking the derivative of the definition of enthalpy (first presented in
Section 2.3.1). Thus,
(6.37)
Expanding the d(PV ) term gives
dH 5 T dS 2 P dV 1 P dV 1 V dP
dH 5 T dS 1 V dP
This is the fundamental property relationship for molar enthalpy.
Can you give a mathematical and/or physical explanation for
going from equation
6.31 to equation 6.32?
The fundamental
property relationships
are composed entirely
of state functions,
which means they are
path-independent.
Here we are applying
the product rule
of calculus: d(xy) =
x dy + y dx
(6.36)
Substituting Equation 6.35 into Equation 6.36 gives
dH 5 T dS 2 P dV 1 dsPVd
FOOd FOr
thOught
6-5
(6.35)
which is termed the fundamental property relationship for molar internal energy.
dH 5 dU 1 dsPVd
The piston-cylinder
device has been used
as a system several
times throughout the
book, beginning with
Examples 1-2 and 3-1.
As usual we regard
changes in the kinetic
and potential energy
of the system as
negligible.
(6.38)
Equations 6.35 and
6.38 are valid for any
pure compounds.
When we model a
mixture of chemical
compounds, there are
more than two degrees
of freedom; H, for example, would depend
upon composition as
well as S and P. Discussion of modeling
of mixtures begins in
Chapter 9.
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248 Fundamentals of Chemical Engineering Thermodynamics
Recall from Section 6.1.1 that a total derivative can be used to relate any intensive property to any other two intensive properties. Expressing H as a function of S
and P:
dH 5
1 −S 2 dS 1 1 −P 2 dP
−H
−H
(6.39)
S
P
Compare Equations 6.38 and 6.39.
Both relate changes in molar enthalpy (dH) to changes in molar entropy (dS)
and pressure (dP).
Equation 6.38 is a thermodynamic relationship between state properties; it is
always valid for any pure substance.
Equation 6.39 is a rule of partial differential calculus; it is always valid for any
pure substance.
The only way Equations 6.38 and 6.39 can both always be true is if the terms
multiplying dS in both equations are equal to each other and the terms multiplying dP in both equations are also equal to each other. Thus,
1 −S 2 5 T
(6.40)
1 −P 2 5 V
(6.41)
−H
These equations are
used in the derivation of Equation 6.19,
which was needed to
solve Examples 6-2
and 6-3.
P
−H
S
In solving the motivational example, it was (∂H/∂P)T we needed to evaluate, not
(∂H/∂S )P or (∂H/∂P)S. However, we will see in Section 6.2.3 that Equations 6.40
and 6.41 can be applied directly to evaluating (∂H/∂P)T using the expansion rule.
Everything we learned about H from Equations 6.38 through 6.41 has its counterpart for U. We can write a total differential for U in terms of S and V as
dU 5
1 −S 2 dS 1 1 −V 2 dV
−U
−U
V
(6.42)
S
By comparing Equations 6.35 and 6.42, we note that
1 −S 2 5 T
−U
1 −V 2 5 2P
−U
(6.43)
V
(6.44)
S
The potential usefulness of the fundamental property relationships for U and H is
straightforward: because U and/or H routinely appear in energy balances, methods of relating them to known information are needed for the solution of real
problems.
Two more state properties of interest in thermodynamics are given here. The
Gibbs free energy (G) is:
G 5 H 2 TS
(6.45)
A 5 U 2 TS
(6.46)
The Helmholtz energy (A) is
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
249
Using derivations analogous to those in Equations 6.36 through 6.38, the fundamental property relationships for the molar gibbs free energy and molar helmholtz
energy are
dG 5 V dP 2 S dT
(6.47)
dA 5 2P dV 2 S dT
(6.48)
The molar Gibbs free energy (G) has not been used substantially up to this point, but
has great importance in modeling phase equilibrium, as discussed starting in Chapter 8.
The molar Helmholtz energy (A) is not often used directly in thermodynamics at
the introductory level, but it is used prominently in the field of statistical mechanics.
For our purposes, the most immediate significance of Equation 6.48 is that it is used
in the derivation of one of Maxwell’s equations, which are discussed in Section 6.2.4.
A brief description of
statistical mechanics is
given in Chapter 7.
6.2.3 the expansion rule
Another useful tool in partial differential calculus is the expansion rule, which says
1−X−Y2 5 1−X−K2 1−Y−K2 1 1−X−L 2 1−Y−L 2
Z
L
Z
K
(6.49)
Z
This is a rule of calculus; it can be applied to any set of variables X, Y, Z, K, and L.
Consider how Equation 6.49 could be applied to the solution of Example 6-2
and the derivation of Equation 6.19. We showed (see Equation 6.30) that solving the problem required evaluation of the partial derivative (∂H/∂P)T . In applying the expansion rule to this example, it is natural to let X = H, Y = P, and
Z = T, transforming the left-hand side into the exact partial derivative we wish
to evaluate:
−L
11 2 1 2
1 −P 2 5 1 −K 2 1−K
2
−P
−L
−P
−H
−H
−H
T
L
T
K
(6.50)
T
In effect, the expansion rule allows us to introduce two new variables, K and L,
into the problem. Section 6.2.2 showed the derivation of known relationships among
molar enthalpy, molar entropy, and pressure. Thus, if we assign K and L in the expansion rule to represent S and P, we can introduce this information into the problem
we’re trying to solve. Equation 6.50 becomes
11 2 1 2
1 −P 2 5 1 −P 2 1−P
−P 2
−S
−P
−H
−H
T
−H
S
T
−S
P
(6.51)
T
And in Section 6.2.2: we derived Equations 6.40 and 6.41, which expressed this
known relationship among H, S, and P as
1 −S 2 5 T
−H
P
While X, Y, Z, K, and
L can be any intensive
variables, a common
and powerful use of
the expansion rule occurs when X is a measure of energy
(U, H, G, A) and K
and L are used to
apply the fundamental
property relationship
for that measure of
energy.
1 −P 2 5 V
−H
S
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250 Fundamentals of Chemical Engineering Thermodynamics
Consequently, substituting Equations 6.40 and 6.41 into Equation 6.51 gives
1T1 2
1 −P 2 5 V 1−P
−P 2
−P
−H
−S
T
T
(6.52)
T
Here we note two additional rules of partial differential calculus:
51
1−X
−X 2
(6.53)
50
1−X
−Y 2
(6.54)
Y
X
Rather than memorizing these, you can probably rationalize them: the change
in X is always equal to the change in X, and the change in X is zero if X is held
constant.
Notice that Equation 6.51 contains the expression (∂P/∂P)T, which is a consequence of the fact that the variables X and K in the expansion rule were both
set equal to the pressure P. Applying Equation 6.53 to Equation 6.51 leads to the
simplification of
1 −P 2 5 V 1 T 1−P2
−H
−S
T
One more step is
required to complete
the derivation of
Equation 6.19.
(6.55)
T
Referring back to the solutions of Examples 6-2 and 6-3, our goal is to derive Equation 6.19, which related JT to measureable properties. The molar entropy is the only
thing on the right-hand side of Equation 6.55 that isn’t a measurable property. It can
be eliminated using one of Maxwell’s equations.
6.2.4 Maxwell’s equations
Recall from Section 6.2.2 that the fundamental property relationship for internal
energy is
dU 5 T dS 2 P dV
One of the rules of partial differential calculus is that when one takes a mixed second derivative with respect to two variables, the order of differentiation does not
matter. Stated mathematically, this rule gives
1
2 1
−2X
−2X
5
−Y −Z
−Z −Y
2
(6.56)
This fact can be applied to the fundamental property relationship for U to learn a
new thermodynamic identity. Equation 6.43 states that
1 −S 2 5 T
−U
V
We can differentiate both sides of Equation 6.43 with respect to V along a path that
keeps S constant, producing
−
−V
31 2 4 1 2
−U
−S
5
V
S
−T
−V S
(6.57)
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
The mixed second derivative of U with respect to S and V can also be found starting
from Equation 6.44 as
−U
5 2P
−V S
1 2
Taking the derivative with respect to S gives
−
−S
31−V 2 4 5 21 −S 2
−U
−P
S
(6.58)
V
V
But because the order of differentiation doesn’t matter, we have
−
−V
31−S 2 4 5 −S− 31−V 2 4
−U
−U
(6.59)
S
V
S
V
Thus,
1−V−T 2 5 21−P−S 2
S
(6.60)
V
While Equation 6.60 is not directly useful in solving the motivational examples, we
note that the process of deriving it can be applied to the fundamental property relationships for H, G, and A. Problem 6-3 asks you to perform the derivations.
The final results are
1−P−T2 5 1 −S 2
−V
S
(6.61)
P
1−P−T2 5 1−V 2
−V
−S
1 −T 2 5 21−P2
−S
V
(6.62)
T
P
(6.63)
T
Collectively, Equations 6.60 through 6.63 are known as Maxwell’s equations.
Equation 6.63 figures directly into the solution of the motivational example. Using
the expansion rule, we obtained Equation 6.55:
1 −P 2 5 V 1 T 1−P2
−H
−S
T
T
Applying Equation 6.63 to Equation 6.55 produces
1 −P 2 5 V 2 T 1 −T 2
−H
−V
T
(6.64)
P
Substituting this result into Equation 6.30 completes the process of deriving Equation 6.19. Thus,
V2T
JT 5 2
1 −T 2
−V
P
CP
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251
252 Fundamentals of Chemical Engineering Thermodynamics
To review, the steps of the derivation are given here.
Recognizing that JT is defined from the same intensive variables (T, P, and H)
as CP, we used the triple product rule to express JT in terms of CP. This step left
−H
the unknown
to resolve.
−P T
−H
We used the expansion rule to evaluate
. The fundamental property rela−P T
tionship reveals that H is a natural function of S and P, so these were the two
variables we introduced into the problem using the expansion rule.
We used one of Maxwell’s equations to eliminate S from the problem, leaving
JT as a function of measurable properties.
1 2
1 2
The next section applies Equation 6.19 to another example of a Joule-Thomson expansion––this time the fluid undergoing the expansion is a liquid.
fOOD fOr
thOught
6-6
If you wash glass or
ceramic dishes in hot
water and then immediately rinse them
with cold water, they
are likely to crack
or break. Why does
this happen, and
how does it relate to
thermodynamics?
The subscript V in this
definition emphasizes
that αV measures
the change of volume with increasing
temperature.
6.2.5 coefficient of thermal expansion and isothermal compressibility
We know from everyday experience that substances typically expand when heated
(there do exist exceptions; liquids and solids that actually decrease in volume when
temperature increases). This phenomenon has great practical importance in design
and construction settings. If a product is expected to experience wide temperature
fluctuations, it must be designed to withstand the resulting volume changes, especially where different materials that expand at different rates are attached to each
other. Physically, this expansion is precisely what the partial derivative expression
(∂V/∂T)P measures––it tells us how much the molar volume of a substance changes
for each degree the temperature is changed at constant pressure. Thus, we define the
coefficient of thermal expansion as
1 −V
V 5
(6.65)
V −T P
1 2
Analogously, we can define a thermodynamic property that measures the effect
of changing pressure on the molar volume of a material, which is the isothermal
compressibility, as
T 5 2
fOOD fOr
thOught
6-7
Why does the definition in Equation 6.66
have a negative sign
in it?
1 2
1 −V
V −P T
(6.66)
The definition of coefficient of thermal expansion can be introduced into Equation 6.19 for
V2T
JT 5 2
JT 5 2
JT 5
1 2
1 −T 2
−V
P
CP
V 2 T sVVd
CP
V
−T
5
sTV 2 1d
−P H CP
(6.67)
For liquids and solids, we have previously used the simple model that V is constant,
which would imply that KT and aV are always zero. In Example 6-4, the coefficient of
thermal expansion is modeled as a constant, but not equal to zero.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
jOuLe-thOMsOn eXPansiOn Of a LiquiD
253
examPle 6‑4
P 5 0.5 atm
T5?
P 5 5 atm
T 5 808F
V 5 0.1 L/mol
Figure 6‑4 Valve examined in Example 6-4.
A liquid enters an adiabatic throttling valve, illustrated in Figure 6-4, at T = 80°F and
P = 5 atm, at which conditions it has V=0.100 L/mol. The exiting liquid has P = 0.5 atm.
For this steady-state process, what is the temperature of the exiting liquid? Assume constant heat capacity CP = 0.1 BTU/mol · °R, constant coefficient of thermal expansion αV =
0.001°R21, and assume the isothermal compressibility is negligible.
SOLUTION:
Step 1 Define a system and simplify the energy balance
This is another Joule-Thomson expansion carried out at steady state; the exact situation
described in Example 6-1 and Equation 6.1. The energy balance, using the valve as the
system, is
H in 5 H out
(6.68)
Here the energy
balance is written
in terms of molar
enthalpy because the
heat capacity is given
on a molar basis.
Step 2 Write a total derivative that relates temperature to pressure and enthalpy
We are trying to find the change in temperature (dT) as the liquid travels through the
valve. The two intensive properties we know the most about are again enthalpy (H ) and
pressure (P).
dT 5
−T
dP
1−H
2 dH 1 1−T
−P 2
(6.69)
H
P
The first term on the right-hand side is zero because the energy balance shows that
enthalpy is constant. We can apply Equation 6.67, which relates the Joule-Thomson
coefficient to measurable properties; for
dT 5
1−P2 dP
−T
PitfaLL
PreventiOn
H
dT 5
V
CP
sTV 2 1d dP
(6.70)
Step 3 A simplifying assumption
We can separate the variables by moving sTV 2 1d over to the left-hand side for
V
dT
5
dP
TV 2 1 CP
(6.71)
The left-hand side is now strictly a function of T and constants (αV being assumed constant). On the right-hand side, we are assuming CP is constant, but what of the molar
volume? Generally speaking (for liquids), liquid volume is not very sensitive to changes
in temperature and pressure. For the moment, let’s assume we can treat V as a constant
with respect to both pressure and temperature, and we will re-evaluate that decision at
the end of the problem.
Temperature is
expressed on an absolute scale in the
definitions of
entropy (dS =
dQrev/T), Gibbs free
energy (G = H – TS),
and Helmholtz energy
(A = U – TS). Temperature (T) must
therefore be expressed on an absolute scale in Equation
6.71, which is derived
from these.
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254 Fundamentals of Chemical Engineering Thermodynamics
Step 4 Solve the integral
The limits of integration for the right-hand side are the known pressures entering and
leaving the valve. For the left-hand side, we must express temperature on an absolute
scale, so 80°F is converted to 540°R. The temperature leaving the valve (Tout) is the unknown we are trying to find.
Integrating Equation 6.71, treating V, CP, and V all as constants, gives
Pout 50.5 atm V
dT
5
dP
CP
Tin 5540 R TV 2 1
Pin 55 atm
#
#
Tout 5Tout
(6.72)
11 2 ln 1 T 2 1 2 5 1C 2sP 2 P d
Tout V 2 1
V
out
V
in
V
in
P
Substituting in known values and applying necessary conversion factors gives
1
9.48 3 1024 BTU and
9.868 3 1023 L · atm
are each equal to
1 Joule.
Touts0.0018R21d 2 1
1
ln
5
0.0018R21
s539.78Rds0.0018R21d 2 1
2 3
4
1
1
L
mol
BTU
0.1
mol ? 8R
2
2
1
2
24
(6.74)
0.1
s0.5 2 5 atmd (6.73)
L
mol
9.48 3 1024 BTU
ln
5 s0.001 R21d
s24.5 atmd
s539.78Rds0.0018R21d 2 1
BTU
9.868 3 1023 L ? atm
0.1
mol ? 8R
3
Touts0.0018R21d 2 1
ln
4
0.1
Touts0.0018 R21d 2 1
3s539.78Rds0.0018R d 2 14 5 24.3 3 10
21
Touts0.0018R21d 2 1
s539.78Rds0.0018R21d 2 1
5 0.99957
The result is Tout = 539.98R = 80.28F.
Example 6-4 examined a liquid expanding from 5 atm and 808F to a final pressure of
0.5 atm. According to this calculation, the liquid temperature only changes by 0.2 degrees as a result of the pressure drop. Examples 6-1 through 6-4 collectively illustrate
that the effect of Joule-Thomson expansion on temperature is much more dramatic
for vapors than liquids (unless the gas is ideal, in which case there is no temperature
change at all). This can be understood mathematically in that the Joule-Thomson
coefficient, according to Equation 6.67, is proportional to V and its derivatives, which
are typically much larger for vapors than liquids. This also means that our decision to
model V as a constant in step 3 is justified; the molar volume V is not going to change
significantly when the temperature changes by just 0.2 K. Problem 6-10 asks you to
estimate this quantitatively.
6.2.6 additional applications of thermodynamic Partial derivatives
Throughout the chapter up to this point, all examples have stemmed from the motivational example, and so involved Joule-Thomson expansion. The modeling techniques discussed throughout this chapter are, however, applicable to any physical
process. To illustrate this, we now examine three different processes: the compression
of a van der Waals gas in a closed (variable volume) system, the heating of a liquid in
a rigid container, and a gas travelling through a reversible nozzle.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
isOtherMaL cOMPressiOn OF a van der WaaLs gas
Five moles of gas are confined in a piston-cylinder device (Figure 6-5). At the beginning
of the process, the gas has T = 300 K and V = 100 L. If the gas is compressed isothermally to a final volume of 15 L, how much work is required, and how much heat is added
or removed? Assume the heat capacity is constant at CV = 30 J/mol · K and that the gas
is modeled by the van der Waals equation of state
P5
RT
a
2
V 2 b V2
255
examPle 6‑5
An alternative to the
Rankine heat engine
is the Stirling cycle,
in which the steps are
isothermal compression (as in this example), isochoric heating,
isothermal expansion,
and isochoric cooling.
with a = 1.40 L2bar/mol2 and b = 0.038 L/mol.
FOOd FOr
thOught
6-8
V1 5 100 L
T1 5 300 K
Published values
(Weast, 1972) of van
der Waals constants include a = 1.408 L2bar/
mol2 for nitrogen and
a = 1.378 L2bar/mol2
for oxygen. Is it reasonable to guess that
air, which is ~79%
nitrogen, would have
a < (0.79)(1.408) +
(0.21)(1.378) <
1.40 L2bar/mol2?
Isothermal
compression
V2 5 15 L
T2 5 300 K
Figure 6‑5 Compression of gas in a piston-cylinder device,
modeled in Example 6-5.
SOLUTION:
Step 1 Define a system and write an energy balance
If we define the five moles of gas as the system, we have a closed system. Applying the
time-independent energy balance and neglecting potential and kinetic energy gives
NsU 2 2 U 1d 5 Q 1 W
(6.75)
Step 2 Evaluate work
The only kind of work in this process is expansion/contraction work, which is known (see
Section 1.4.3) to be equal to –P dV. The initial and final volumes of the gas are known, so
we can set up the integral:
WEC 5 2
#
V515 L
V5100 L
(6.76)
P dV
Here, P is the pressure of the gas, which is given by the van der Waals equation of state:
WEC 5 2
#
V515 L
V5100 L
P is the pressure
opposing the motion.
Here the gas is being
compressed, so the
pressure opposing the
motion is the pressure
of the gas itself.
3VRT2 b 2 Va 4 dV
2
(6.77)
Recognizing that V and V are two different quantities, we relate them to each other as
V 5 NV
(6.78)
Because N is constant in this case, we can write
dV 5 N dV
(6.79)
PitfaLL
PreventiOn
Don’t confuse V and
V. The limits of integration are either
V = 100 L to V = 15 L
if you integrate with
respect to total volume, or V = 20 L/mol
to V = 3 L/mol if you
integrate with respect
to molar volume.
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256 Fundamentals of Chemical Engineering Thermodynamics
and insert this into Equation 6.77 for
#
WEC 5 2N
This is an isothermal
compression. T is constant in this process,
and can be moved outside the integral.
V53 L/mol
V520 L/mol
3VRT2 b 2 Va 4 dV
(6.80)
2
1V 2 b2 1 N1V 2 Va 2
V2 2 b
WEC 5 2sNRTd ln
a
1
2
1
Plugging in known values gives
1
2
bar ? L
WEC 5 2s5 mold 0.08314
s300 Kd ln
mol ? K
1 s5 mold
1
1
L
mol
L
20 2 0.038
mol
L2 ? bar
L2 ? bar
1.4
mol2
mol2
2
L
L
3
20
mol
mol
1.4
1
3 2 0.038
(6.81)
2
J
mol ? K
WEC 5 s237.9 bar ? L 1 2.0 bar ? Ld
bar ? L
0.08314
mol ? K
8.314
2
2
5 24, 000 J
We now know the work, but in order to find the heat (Q), we need to close the energy balance. This means we need to calculate the change in specific internal energy (U 2 2 U 1).
Step 3 Write a total derivative that describes U
We can write a total derivative expression that relates dU to changes in any other two
intensive properties. The two we know the most about are temperature and molar volume.
dU 5
1 −V 2 dV 1 1 −T 2 dT
−U
−U
(6.82)
V
T
Because this is an isothermal process, the dT term can be eliminated (dT = 0) for
dU 5
1 −V 2 dV
−U
(6.83)
T
Thus, solution of the problem requires us to relate the partial derivative (∂U/∂V )T to
known properties. This requires several steps, but can be done using a strategy analogous
to the one illustrated in Sections 6.2.2 and 6.2.3. We can use the expansion rule to find an
expression for (∂U/∂V )T, but what two parameters should we introduce into the problem
(in other words, what should we choose as L and K in Equation 6.49)? We can answer
this question by exploring the fundamental property relationship for U.
Step 4 Apply fundamental property relationship for U
Equation 6.35 gives the fundamental property relationship for U:
dU 5 T dS 2 P dV
We learned in Section 6.1.1 that a total derivative can be used to relate any intensive property to any other two intensive properties. Expressing U as a function of S and V gives
dU 5
1 −S 2 dS 1 1 −V 2 dV
−U
−U
V
(6.84)
S
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
257
The only way Equations 6.35 and 6.84 can both be true is if the dS terms in both equations
are equal to each other and the dV terms in both equations are also equal to each other:
1 −S 2 5 T
(6.85)
1 −V 2 5 2P
(6.86)
−U
V
−U
S
Step 5 Apply expansion rule
The expansion rule is
−X
−K
−X
−L
51 2 1 2 11 2 1 2
1−X
2
−Y
−K
−Y
−L
−Y
Z
L
Z
K
(6.87)
Z
It is natural to define X = U, Y = V, and Z = T, since that transforms the left-hand side
into the partial differential we seek to evaluate. If we define K = S and L = V, we can
make use of Equations 6.85 and 6.86 for
1 −V 2 5 1 −S 2 1−V2 1 1 −V 2 1−V 2
−U
−U
−S
V
T
−U
−V
S
T
(6.88)
T
And introducing Equations 6.85 and 6.86 into Equation 6.88 gives
1 −V 2 5 T 1−V2 1 s2Pd1−V2
−U
−S
T
−V
T
(6.89)
T
We can eliminate the specific entropy from the right-hand side by applying one of Maxwell’s
equations. We also note that the derivative of V with respect to V is 1 (see Equation 6.53). Thus,
1 s2Pd
1 −V 2 5 T 1−P
−T 2
−U
T
(6.90)
V
The right-hand side is now entirely composed of measurable properties, and can be evaluated.
Step 6 Evaluate partial derivative of pressure for van der Waals equation
A partial derivative expression that relates P, V, and T to each other can be evaluated
if we know an equation of state that describes the material well; in this case the van der
Waals equation.
Differentiating the van der Waals equation with respect to T gives:
1 2
−P
R
5
−T V V 2 b
(6.91)
Step 7 Evaluate original total derivative
The purpose of steps 4 through 6 was to evaluate Equation 6.83, which is the original
total derivative relating dU to changes in temperature (dT ) and specific volume (dV ).
Applying Equation 6.90 to Equation 6.83 gives
dU 5
2 P4 dV
1 −V 2 dV 5 3T 1−P
−T 2
−U
T
The simplification that
occurs between Equations 6.88 and 6.89, in
which U is eliminated
from the right-hand
side, is neither “luck”
nor a fluke. We know
U is related to S and
V through the fundamental property relationship, and defining
K = S and L = V
introduces this knowledge into the solution
of a problem.
In this differentiation,
V is being held constant, so the term a/V 2
is a constant, and its
derivative is zero.
(6.92)
V
In order to integrate the right-hand side, we need to express everything as a function of
V. An expression for (∂P/∂T )V is known through Equation 6.91, and P can be found from
the equation of state itself. Substituting these into Equation 6.92 gives
5 1V R2 b2 2 1VRT2 b 2 Va 26 dV
dU 5 T
dU 5
a
dV
V2
2
(6.93)
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258 Fundamentals of Chemical Engineering Thermodynamics
FOOd FOr
thOught
6-9
Internal energy for
real gases is (or at
least can be) dependent upon pressure.
So, why isn’t there
also a dP term in this
equation?
This shows mathematically a difference between ideal and real gases. For an ideal gas,
specific internal energy is only a function of temperature; U 2 2 U 1 would be 0 for an
isothermal ideal gas process. Equation 6.93 reflects mathematically that compressing a
real gas into a smaller volume––even if it is done isothermally––affects the internal energy due to intermolecular interactions.
Step 8 Integrate the total derivative
Integrating Equation 6.93 from the initial state to the final state gives
# dU 5 #
V53 L/mol
a
V520 L/mol V
U2 2 U1 5 2
1V 2 V 2 5 2
a
a
2
1
1
L ? bar
U 2 2 U 1 5 20.40
mol
2
1
1
1.4
2
dV
(6.94)
L2 ? bar
L2 ? bar
1.4
2
mol
mol2
2
L
L
3
20
mol
mol
J
mol ? K
bar ? L
0.08314
mol ? K
8.314
2
5 240
2
J
mol
(6.95)
Step 9 Close energy balance
Inserting the known values for U and W into the energy balance gives
NsU 2 2 U 1d 5 Q 1 W
1
s5 mold 240
(6.96)
2
J
5 Q 1 24, 000 J
mol
Q 5 224, 200 J
For an ideal gas, U is a function of temperature only, and an isothermal compression
or expansion would have ΔU = 0 and Q = 2W. We have noted previously that, for
real gases, U is also a function of pressure and volume. In Example 6-5, we saw for
the first time how pressure or volume dependence can be modeled.
The next example examines a case in which the isothermal compressibility and
coefficient of thermal expansion of a liquid are both known and assumed to be
constants.
examPle 6‑6
You’ve likely heard
the basic lab safety
tip “Don’t heat a
closed container.” This
problem quantifies
an effect of heating a
closed, rigid container.
heating a LiquiD in a rigiD cOntainer
The tank of a water heater (Figure 6-6) has V = 80 L and is initially sealed and full of
water at T = 130°F and P = 1 atm. The water heater is designed to maintain the water at
this temperature, but due to a fault in the temperature sensor, the heating element comes
on and remains on. The temperature of the water increases, but there is no space into
which the water can expand. The tank has a pressure relief value that is designed to open
at P = 10 atm. At what water temperature will this occur?
For this example assume the isothermal compressibility and coefficient of thermal
expansion of water are constant at V = 2.90 3 10248F21 and KT = 4.50 3 1025 atm21.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
P 5 1 atm
T 5 1308F
V 5 80 L
259
P 5 10 atm
T5?
V 5 80 L
Figure 6‑6 Isochoric heating of a water heater.
SOLUTION:
Step 1 Define a system and process
The system is the water in the tank. The description of the tank says that the water has
“no room to expand,” so we will model this as a rigid, constant-volume system. Since the
tank is described as “sealed ,” this is also a closed system.
Since our goal is to find the temperature at which the relief valve will open, the process begins when the heating elements turns on, and ends when the pressure in the tank
reaches exactly P = 10 atm.
Step 2 Write a total derivative that describes changes in temperature
The problem asks us to find an unknown temperature. Changes in temperature (dT) can be
related to changes in any other two intensive properties through a total derivative. In this case,
the two variables we know the most about are volume (the container is rigid) and pressure
(the initial and final pressure are both known). So we write the total derivative expression as
dT 5
1−V2 dV 1 1−P2 dP
−T
−T
(6.97)
V
P
The total volume (V) is constant in this rigid container. The mass (M) is also constant;
this is a closed system. Therefore, the specific volume (V = V/M) must also be constant.
Thus, the volume term of Equation 6.97 can be eliminated (dV = 0).
dT 5
1−P2 dP
−T
(6.98)
V
Step 3 Evaluate the partial differential (∂T/∂P )V
To evaluate (∂T/∂P)V, our first thought might be to apply an equation of state, as we did
when we found (∂P/∂T)V for the gas in Example 6-5. We do not have an explicit equation
of state for this liquid, but we do have two pieces of given information about the relationship among P, V, and T. The isothermal compressibility and the coefficient of thermal
expansion are both known. By definition,
V 5
1 2
1 2
1 −V
1 −V
and T 5 2
V −T P
V −P T
that we know the
value of a different
partial derivative that
relates the same three
variables to each other
is a hint that we might
try applying the triple
product rule.
We can relate the known information to (∂T/∂P)V through the triple product rule:
−Y
−Z
5 21
1−X
−Y 2 1 −Z 2 1 −X 2
Z
X
Y
which for T, P, and V becomes
−P
5 21
1−T
−P 2 1 −V 2 1 −T 2
−V
V
T
We want to know
(∂P/∂T)V. The fact
(6.99)
P
We apply Equation 6.29 to invert the (∂P/∂V)T term, so that it mirrors the definition of V.
1−T
−P 2 1 −T 2
5 21
−V
1 −P 2
−V
V
P
(6.100)
T
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
260 Fundamentals of Chemical Engineering Thermodynamics
Substituting the definitions of V and T into Equation 6.100 yields
1−P2 sV d
−T
Equation 6.101 is
not specific to this
problem; we assumed
nothing in its derivation so it’s always true.
Here, however, it’s
particularly useful, because the isothermal
compressibility and
coefficient of thermal
expansion are both
known and modeled
as constant.
V
V
s2VTd
5 21
5
1−T
−P 2
T
V
(6.101)
V
Thus, in this example, (∂T/∂P)V is equal to a constant.
Step 4 Solve integral
The purpose of step 3 was to convert (∂T/∂P)V into a form that can be integrated so that
Equation 6.98 can be solved. Thus,
dT 5
dP 5
dP
1−T
−P 2
T
V
#
T2
dT 5
T1 51308F
T2 2 130 8 F 5
#
P2 510 atm T
P1 51 atm
T2 2 T1 5
(6.102)
V
T
V
V
dP
sP2 2 P1d
4.50 3 1025 atm21
s10 2 1atmd
2.90 3 1024 8 F21
(6.103)
The result is T2 = 131.48F.
You probably already knew that when liquids are heated in a confined space, there
is the danger that if the liquid boils, a rapid, extreme pressure buildup can result.
The previous example shows that even if the liquid does not boil, heating it in a
rigid container can lead to a dramatic pressure increase. The volume of water is not
very sensitive to temperature—heating water from 1308F to 131.48F at a constant
pressure of 1 atm would only cause the volume to increase by about 0.04%. But if
this fractional expansion is not permitted to occur, the pressure builds at a rather remarkable rate––in Example 6-6 a 1.48F temperature increase accompanies an orderof-magnitude change in pressure. This example illustrates why a typical home water
heater has a built-in expansion tank.
The final example looks at a gas travelling through a reversible nozzle, and in the
process, derives another useful partial derivative expression, relating molar entropy
to heat capacity.
examPle 6‑7
gas traveLLing thrOugh a reversibLe nOzzLe
FOOd FOr
thOught
6-10
A gas flows through an adiabatic nozzle (Figure 6-7) at steady state, entering at P = 5 bar
and T = 400°C, and leaving at P = 1 bar. Assuming the nozzle is reversible, what is the
temperature of the exiting gas? Assume the gas has a constant heat capacity of CP =
40 J/mol · K and follows the equation of state:
Is this a realistic equation of state?
V5
RT
1 aTP
P
where a = 1.00
cm3
mol ? bar ? K
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
P 5 5 bar
T 5 4008C
261
P 5 1 bar
T5 ?
Figure 6‑7 Nozzle modeled in Example 6-7.
SOLUTION:
Step 1 Define a system and write an entropy balance
The nozzle itself is the system. The inlet conditions and outlet pressure are identical to
Example 3-4, but in that problem, the outlet temperature was also known, and we used
an energy balance to find the unknown outlet velocity. Here, our first instinct might be to
attempt to apply the energy balance, but the outlet temperature and outlet velocity are
both unknown, so the energy balance would be one equation in two unknowns. Writing
the entropy balance is more useful here because (1) the unknown velocity is not a factor
in the entropy balance, and (2) we can directly apply the fact that the nozzle is well approximated as reversible.
For a steady-state, adiabatic, and reversible process, the entropy balance simplifies to
S in 5 S out
(6.104)
Step 2 Write a total derivative expression relating molar entropy to measurable properties
We know the inlet and outlet pressure, and the outlet temperature is what we’re trying to
find, so it’s natural to relate S to P and T through the total derivative. Thus,
dS 5
1−P2 dP 1 1−T2 dT
−S
−S
T
(6.105)
P
Step 3 Convert Equation 6.105 into an expression that can be integrated
The left-hand side is simply dS, which could be integrated immediately. We can use one
of Maxwell’s relations (Equation 6.63) to re-express (∂S/∂P)T as a function of measurable properties:
dS 5 2
1 −T 2 dP 1 1−T2 dT
−V
−S
P
(6.106)
PitFaLL
PreventiOn
You may be tempted
to cross out the term
−V
dP, thinking “if
−T P
P is being held constant, then dP must
be zero.” But if P is
held constant when
one partial derivative is calculated, that
doesn’t mean P is held
constant everywhere
in the entire problem.
The given information
clearly shows pressure is changing, so we
shouldn’t expect an
integral with respect
to dP to equal 0.
1 2
P
While there was no clear way to evaluate (∂S/∂P )T, the expression (∂V/∂T )P is readily
evaluated using the known equation of state. Taking the derivative with respect to T
(holding P constant) gives
1 −T 2 5 RP 1 aP
−V
(6.107)
P
And substituting this into Equation 6.106 gives us an expression that can be integrated as
dS 5 2
1RP 1 aP2 dP 1 1−T2 dT
−S
(6.108)
P
Thus, the dS and dP terms are now readily integrated. What of the dT term?
Step 4 Relate (∂S/∂T )P to the known heat capacity CP
While there is no simple way to evaluate (∂S/∂T )P using the equation of state––as there
was for (∂S/∂P)T––one can relate (∂S/∂T )P to the heat capacity in a straightforward way.
By definition, CP = (∂H/∂T )P.
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262 Fundamentals of Chemical Engineering Thermodynamics
Here, we use the expansion rule with X =
H, Y = T, and Z = P
(to make the left-hand
side equal CP), K = S,
and L = P (which
allows us to make use
of the fundamental
property relationship
for H).
Applying the expansion rule gives
1 −T 2 5 1 −S 2 1−T2 1 1 −P 2 1−P
−T 2
−H
−H
−S
P
−H
P
S
P
(6.109)
P
The last term is 0 (Equation 6.54). We apply Equation 6.40 and the definition of heat
capacity to give
1 −T 2 5 C 5 T 1−T2
−H
−S
(6.110)
P
P
P
Applying this to Equation 6.108 gives
dS 5 2
1RP 1 aP2 dP 1 T dT
CP
(6.111)
Step 5 Integrate Equation 6.111
Integrating from the state of the inlet gas to the state of the outlet gas gives
FOOd FOr
thOught
6-11
Would an irreversible
nozzle produce higher
or lower kinetic energy than a reversible
one, for the same inlet
stream and the same
pressure drop?
#
S out
dS 5 2
#
Pout 51 bar
Pin 55 bar
S in
S out 2 S in 5 2R ln
1RP 1 aP2 dP 1 #
Tout
CP
Tin 5673.15 K
T
dT
(6.112)
1 P 2 2 a2 sP 2 P d 1 C ln 1 T 2
Pout
2
out
Tout
2
in
P
in
in
Step 6 Substitute known values
The entropy balance showed that S out=S in; thus, the left-hand side is 0. Tout is the only
unknown on the right-hand side:
1
0 5 2 8.314
3
1
0 5 13.38
2 1 2
J
1 bar
ln
2
mol ? K
5 bar
J
mol ? K
bar ? cm3
83.14
mol ? K
8.314
2
1
1.00
cm3
bar ? mol ? K
2
2
s1bar2 2 25bar2d
1 molJ? K2 ln 1673.15 K2
Tout
1 40
1
2 1
Tout
J
J
J
1 1.20
1 40
ln
mol ? K
mol ? K
mol ? K
673.15K
2
The result is Tout = 467.5 K or 194.48C.
Step 4 of Example 6-7 demonstrated that CP, which is by definition a partial derivative of molar (or specific) enthalpy, also can be related to a partial derivative of molar
(or specific) entropy:
1−T2 5 T
−S
No assumptions were
made in the derivation
of Equation 6.113.
Equations 6.113 and
6.114 are both completely general.
CP
(6.113)
P
An analogous derivation can be used to arrive at the following equation for CV:
1−T2
−S
V
5
CV
T
(6.114)
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
263
6.2.7 summary of useful Partial differential expressions
baSiC laWS oF CalCuluS
The total derivative can be used to relate any INTENSIVE property of a pure, homogeneous phase to any other two INTENSIVE properties.
dX 5
−X
dY 1 1 2 dZ
1−X
2
−Y
−Z
Z
Y
The expansion rule can be used to introduce two new properties (K and L) into a
problem, helping one to express (∂X/∂Y)Z again as a function of other properties
that are better known, or easier to find, in the problem at hand.
−X
−K
−X
−L
51 2 1 2 11 2 1 2
1−X
2
−Y
−K
−Y
−L
−Y
Z
L
Z
K
Z
The triple product rule can be used to re-express (∂X/∂Y)Z in terms of other partial
derivatives that relate X, Y, and Z.
−Y
−Z
5 21
1−X
2
1
2
1
−Y
−Z
−X 2
Z
X
Y
These fundamental identities are often helpful in simplifying partial differential equations:
In modeling extensive
properties of a pure
substance, a threeterm total derivative
expression would be
needed, because the
quantity of mass is one
additional degree of
freedom beyond the
two that come from
intensive properties.
1
5
1−X
2
−Y
−Y
1−X
2
51
1−X
−X 2
50
1−X
−Y 2
Z
Z
Y
X
FunDamenTal ProPerTy relaTionShiPS
The molar internal energy, enthalpy, Helmholtz energy, and Gibbs free energy can
be related to other intensive properties of any PURE compound through the fundamental property relations.
dU 5 T dS 2 P dV
dH 5 T dS 1 V dP
dA 5 2P dV 2 S dT
dG 5 V dP 2 S dT
DeFiniTionS
The constant-pressure heat capacity is defined as
CP 5
1 −T 2 5 T 1−T2
−H
−S
P
P
When the heat capacity is valid at ideal gas
conditions specifically,
it is denoted with an
asterisk, CP* or CV*.
The constant-volume heat capacity is defined as
CV 5
1 −T 2 5 T 1−T2
−U
−S
V
V
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264 Fundamentals of Chemical Engineering Thermodynamics
The Joule-Thomson coefficient is defined as
JT 5
52
1−T
−P 2
V2T
1 −T 2
−V
P
CP
H
The coefficient of thermal expansion is defined as
V 5
1 2
1 −V
V −T P
The isothermal compressibility is defined as
T 5 2
1 2
1 −V
V −P T
maxWell’S equaTionS
These are often helpful in eliminating molar entropy (S ) from a problem, leaving it
completely in terms of measurable properties (P, V, T ).
−T
1−V
2 5 21−P
−S 2
S
V
1 2 1 2
−V
−T
5
−P S
−S P
51 2
1−P
−T 2
−V
−S
V
1 −T 2
−V
P
T
1−P2
52
−S
T
6.3 heat capacity and residual Properties
Examples 6-2 through 6-7 illustrate that we can solve a lot of practical problems in
the absence of data if we have two things: an equation of state that provides a good
estimate of the properties of the substance, and an accurate measure of the heat
capacity. This section takes a closer look at the heat capacity, specifically noting the
following two things.
1. Heat capacity can itself be modeled using an equation of state.
2. Commonly, the heat capacity can be considered known at ideal gas conditions
but not at higher pressures. We will show how “real gas” problems can be solved
from this information.
6.3.1 distinction between real and ideal gas heat capacity
Section 2.3.2 introduced the heat capacity, and described how the constant-pressure
heat capacity (CP) can be measured over a range of temperatures from a single experiment. At this point we will consider the relationship between heat capacity and
pressure, starting with an example that examines steam.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
265
estiMating Ĉ P OF steaM FrOM steaM tabLes
examPle 6‑8
Estimate the constant pressure heat capacity (ĈP) of steam at the various temperatures
and pressures shown in Table 6-1, using only the data shown in the steam tables.
The definition of a derivative df(x)/dx is:
SOLUTION:
Step 1 Compare data to definition of heat capacity
By definition, CP 5 (∂H/∂T)P, or in this case, because the steam tables are on a mass basis,
ĈP 5 s−Ĥ/−TdP. Taking P 5 0.1 bar and T 5 350°C as an example, we note that the steam
tables contain data for Ĥ at P 5 0.1 bar and temperatures of T 5 300°C and T 5 400°C.
Calculating ∆Ĥ/∆T between these points gives us a reasonable estimate of (∂Ĥ/∂T)P,
since we are measuring the change in specific enthalpy with respect to the change in
temperature, while holding the pressure constant at P 5 0.1 bar (in which we are interested). We would expect the estimate to become more accurate if based on data points
that were closer to 350°C (e.g., 340°C and 360°C), but that data is not available in these
steam tables.
limDx → 0
f sx 1 Dxd 2 f sxd
Dx
In this numerical
estimate of (∂Ĥ/∂T )P,
you can think of 3008C
as x and 1008C as Dx,
which means x 1 Dx 5
4008C. The specific
enthalpy at 3008C,
3076.7 kJ/kg, is f(x),
and the specific enthalpy at 4008C, 3279.9
kJ/kg, is f(x 1 Dx).
Step 2 Look up data
ĈP <
Ĥ400 2 Ĥ300
DĤ
5
5
DT
400 2 3008C
3279.9 2 3076.7
kJ
kg
400 2 3008C
5 2.032
kJ
kg8C
(6.115)
The remaining values shown in Table 6-1 were estimated using the same process.
tabLe 6-1 ĈP for steam at various temperatures and pressures, as estimated
using steam tables. All values are in kJ/kg · K.
P = 0.1 bar
P = 0.5 bar
P = 10 bar
P = 50 bar
T = 350°c
2.032
2.036
2.128
2.710
T = 550°c
2.166
2.167
2.195
2.322
T = 750°c
2.307
2.308
2.320
2.374
Plotting all of the data
for P = 0.1 bar and
measuring the slope
of the curve at T =
350°C is a more timeconsuming, but more
rigorous, method of
estimating s−Ĥ/−TdP.
Example 6-8 illustrates the estimation of heat capacity from discrete data points.
The values in Table 6-1 demonstrate that the heat capacity of steam is a function
of temperature; the relationship between CP and T has also been explored previously (see Example 2-5). The table proves, however, that CP is also dependent
upon pressure.
Recall that real gases behave like ideal gases at low pressures, as first discussed
in Section 2.3.3. A critical observation from Table 6-1 is that the estimates for
P = 0.1 bar and P = 0.5 bar (low pressures) are practically identical, while the data
for 10 bar and 50 bar (high pressures) are noticeably different.
Table 6-1 illustrates some facts that have been mentioned previously:
For ideal gases, enthalpy is only a function of temperature, and consequently heat
capacity is also only a function of temperature.
For real gases, heat capacity depends upon both temperature and pressure.
Section 2.3.2 outlined how CP could be measured experimentally in a constantpressure vessel. By adding heat at a constant rate and monitoring the temperature
of the material, one can measure CP over a large range of temperatures in a single
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266 Fundamentals of Chemical Engineering Thermodynamics
experiment. However, in general, the measured values of CP would only be valid at
the specific pressure for which the experiment was conducted.
At low pressures, however, real gases behave like ideal gases; CP does not depend upon pressure. Consequently, if we measure CP versus T at a low pressure, we
can apply this value of CP to any other low pressure. Thus, the numbers listed in
the first two columns (P = 0.1 bar and P = 0.5 bar) of Table 6-1 can be considered
estimates of the heat capacity of steam at ideal gas conditions. We have previously
defined the ideal gas heat capacity (CP*) of a substance as the heat capacity under
conditions at which the substance can be reasonably modeled as an ideal gas.
When one is solving problems or designing processes that involve high
pressures, it is relatively common that an ideal gas heat capacity is known, but a
value of the heat capacity at the exact pressure of interest is not available. Indeed,
Appendix D of this book shows the ideal gas heat capacity for a number of common
chemical compounds, as a function of temperature. The next sections explore how
we can make use of a known ideal gas heat capacity, even at elevated pressures.
6.3.2 Motivation for the residual Property
By now a turbine is a unit operation that is recognizable as a familiar and important
physical system. Example 6-9 examines a turbine, and uses the common scenario
that was posed in the previous section: the heat capacity is known only for ideal
gas conditions. The example follows the problem-solving strategy outlined in
Section 6-2.
examPle 6‑9
exaMPLe in Which C P* is KnOWn
Methane enters a turbine (Figure 6-8) at T = 600 K and P = 10 bar and leaves at T = 400 K
and P = 2 bar. How much work is produced for each mole of gas? Use the following data:
van der Waals constants for methane are a = 0.2303 Pa-m6/mol2 and b = 4.306 3 1025 m3/mol.
C P* for methane is given in Appendix D.
P 5 10 bar
T 5 600 K
P 5 2 bar
T 5 400 K
Ws /n 5 ?
Figure 6‑8 Turbine described in Example 6-9.
SOLUTION:
Step 1 Define a system and write an energy balance
Lacking any contrary information, we apply the usual assumptions we’ve used for turbines (steady-state, adiabatic, negligible kinetic and potential energy), arriving at the
equation given in Table 3-1:
·
WS
· 5 Ĥout 2 Ĥin
m
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
267
For this example, however, it is more convenient to express the energy balance on a
molar basis. If we call the state of the entering gas “1” and the exiting gas “2,” the energy
balance becomes
·
WS
5 H2 2 H1
(6.116)
n·
Step 2 Write a total derivative expression for dH
The left-hand side of Equation 6.116 is exactly what we are trying to find—work produced per mole of gas—so solving the problem requires evaluating H 2 2 H 1. The normal
way to do this is to integrate dH:
#
T2, P2
T1, P1
dH 5 H 2 2 H 1
(6.117)
So as we’ve done throughout this chapter, we look to use a partial derivative expression
to relate what we want to know (dH) to the two intensive properties we know the most
about:
dH 5
dT 1 1 2 dP
1−H
−T 2
−P
−H
P
(6.118)
T
Or, applying the definition of heat capacity:
dH 5 CP dT 1
1 −P 2 dP
−H
(6.119)
T
Step 3 Relate (∂H/∂P)T to measurable properties
Applying the expansion rule with X = H, Y = P, Z = T, K = S, and L = P:
1 −P 2 5 1 −S 2 1−P2 1 1 −P 2 1−P
−P 2
−H
−H
T
−S
P
−H
S
T
(6.120)
T
Introducing Equations 6.40 and 6.41 into Equation 6.120 gives
1 −P 2 5 T 1−P2 1 V 1−P
−P 2
−H
−S
T
T
(6.121)
T
Entropy can be eliminated from the right-hand side by applying one of Maxwell’s equations (6.63) and (∂P/∂P)T is simply equal to 1 (Equation 6.53):
1 −P 2 5 2T 1 −T 2 1 V
(6.122)
3
(6.123)
−H
−V
T
P
Thus, Equation 6.121 becomes
dH 5 CP dT 1 V 2 T
1 −T 2 4 dP
−V
P
We assumed nothing in deriving Equation 6.123; it is a valid relationship among H, T,
and P for any process involving a pure compound. In Example 6-9, however, Equation 6.123 illustrates the barrier we face in solving the problem; the quantity CP dT
cannot be integrated unless CP is known as a function of temperature. It is indeed
known for the ideal gas state, but assuming that methane behaves like an ideal gas
is questionable at P 5 10 bar. A residual property, or residual, is a quantitative
measure of the difference between the real material and an ideal gas at a given temperature and pressure. Residual properties would apply to Example 6-9 in that we
X, Y, and Z are
assigned to transform
the left-hand side into
the very partial derivative we are trying to
evaluate. K and L are
chosen because the
fundamental property
relationship expresses
H as a function of S
and P, so we introduce S and P into the
problem.
We will use Equation
6.123 in developing
the enthalpy residual
function, and will
complete the example
(see Example 6-9
revisited) using
residual functions.
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268 Fundamentals of Chemical Engineering Thermodynamics
What we call “residual
properties” are, in
some books, called
“departure functions,”
because they measure
how much a real substance departs from
ideal gas behavior.
can use the ideal gas heat capacity to calculate what the change in enthalpy would be
if methane were an ideal gas at these conditions, and then use residual properties to
quantify the difference between the real methane and an ideal gas.
Residual properties and their use are introduced in Section 6.3.3. First, we note
that the previous discussion is intended to show a motivation for residual properties,
but it shouldn’t be misunderstood. We are not claiming Equation 6.123 is impossible
to solve without them. Indeed, two other solution strategies are possible using what
we’ve learned up to this point; you might try to identify at least one of them yourself
before reading on. A brief consideration of these alternative solution methods will
further illustrate why residual properties are useful.
One Possible Solution Strategy We saw in Section 6.2 that, by applying principles
of partial differential calculus, we can derive relationships between a host of intensive properties, such as U, H, S, P, V, and T. So it seems logical to look for a relationship between CP and P and, indeed, it can be proved that
The solution strategy
described here sounds
cumbersome, but we
don’t avoid it because
it’s difficult. We avoid
it because there’s a
valid mathematical
reason to expect residual functions will
provide a more accurate answer.
1 −P 2
−CP
5 2T
T
1−T 2
−2V
(6.124)
2
P
Since the right-hand side is entirely a function of measurable properties, it can be
evaluated using an equation of state. We could estimate the heat capacity of methane at 600 K and 10 bar by finding the ideal gas heat capacity at 600 K, assuming
this is valid at some low pressure (e.g., P = 0.1 bar) and then use Equation 6.124 to
quantify the change in CP as the pressure is increased from P = 0.1 bar to P = 10 bar
at constant temperature.
While logical, this strategy has the drawback that Equation 6.124 contains a second derivative. One must always keep in mind that equations of state are models
that provide estimates of real physical properties. While an equation of state might
describe the P-V-T relationships for a compound quite accurately, uncertainties and
errors tend to get magnified by operations such as differentiation, as illustrated in
Figure 6-9 and Figure 6-10. Let us imagine that function A (solid line) in Figure 6-9
represents real data and function B (dashed line) represents a model that is used to
estimate the data. At a glance, we would probably say the agreement is excellent, and
60
50
B
40
A
30
20
10
0
0
10
20
30
40
50
60
Figure 6‑9 Two functions are shown. For any given value of
x, the values of y for the two functions differ by less than 3% in
magnitude.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
269
0.5
B
0
0
10
20
30
40
50
60
A
20.5
21
21.5
22
22.5
23
Figure 6‑10 The derivative curves for the two functions shown
in Figure 6-9.
a close inspection of the numerical values reveals that the “model value” of y (curve B)
is always within 3% of the “real y” (curve A) for any given x. However, if we look
at the slopes of curves A and B, we find values of dy/dx become noticeably different
as x increases. Figure 6-10 shows that between x = 46 and x = 52 the derivatives are
actually different in sign. Thus, the model fits “y” much more accurately than it fits
“dy/dx.” The uncertainty of the model would be compounded further if we took a
second derivative with respect to x.
Residual properties, which are introduced in Section 6.3.3, allow us to solve
problems like Example 6-9 and require only taking the first derivatives of equations
of state—never a second derivative.
A Second Possible Strategy for Solving Example 6-9 Recognizing that H is a state
property, you might have proposed the idea of constructing a hypothetical path for
the process in which the temperature only changes at low pressure. For example,
consider the path shown in Figure 6-11.
Steps 1 and 3 are isothermal, so the dT term in Equation 6.123 is 0, regardless of
the value of CP.
Step 2 is carried out at a low pressure of P = 0.1 bar. At these conditions, the
known ideal gas C P* can be assumed valid. Because it’s an isobaric step, the dP
term in Equation 6.123 is 0 here.
P 5 10 bar
T 5 600 K
Actual path
Step 1—isothermal
P 5 2 bar
T 5 400 K
Equation 6.123 is a
general relationship
among H, T, and P; we
can apply it individually to find the change
in molar enthalpy for
each of the three steps
in Figure 6-11.
Step 3—isothermal
P 5 0.1 bar Step 2—isobaric P 5 0.1 bar
T 5 600 K
T 5 400 K
Figure 6‑11 A hypothetical path useful for
analyzing the process in Example 6-9.
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270 Fundamentals of Chemical Engineering Thermodynamics
This solution method is logical, and we will find that it is actually very similar to
solving the problem using residual properties. However, Figure 6-11 is specific to
the process in Example 6-9. Residual properties place this approach into a general
framework that can be applied to any number of problems.
The concept of a state
property was first
introduced in Section 2.2.4, and the
Gibbs phase rule
was introduced in
Section 2.2.3.
This discussion focuses on H, U, and S,
because those are the
properties that were
important in the problems we’ve solved up
to this point. In Chapter 8, the molar Gibbs
free energy residual
function G – Gig will
have great importance.
Like the “hypothetical reversible turbine”
first introduced in
Example 4-7, H 1ig and
H 2ig are theoretical
constructs that don’t
exist in the real world,
but are useful in solving problems.
6.3.3 Definition of residual Properties
Recall that a state property is dependent only upon the current state of the system
and (as shown with the Gibbs phase rule) a pure, homogeneous phase has two
degrees of freedom. If we specify two intensive properties, such as temperature and
pressure, then all of the others—such as H, U, and S—have a unique value that is
always valid for a compound at that particular T and P.
Section 2.2.3 introduced the ideal gas model, and we have since established
methods of quantifying how these same state properties (H, U, S, P, T, and V ) are
interrelated for an ideal gas. While the ideal gas model only describes real gases at
low pressures, the equations can be applied at any conditions. Thus, even though the
real world contains no such thing as an ideal gas at P = 50 bar and T = 500 K, we
can use the ideal gas model to evaluate what H, U, S, or V would be if an ideal gas
existed at those conditions. We denote such hypothetical ideal gas properties as H ig,
U ig, S ig, and V ig.
A residual property is defined as the difference between a real property (e.g., H) for the
real compound and the property for a hypothetical ideal gas that exists at the same temperature and pressure (e.g. , H ig).
Thus, the residual property for molar enthalpy is written H 2 H ig. It is termed a
“residual” because it measures what remains after the value of the property in the
ideal gas state is subtracted.
Example 6-9 in the previous section examined a turbine, and solution of the
problem required us to find the difference in molar enthalpies of the inlet (H1) and
outlet (H 2) streams.
Changes in molar enthalpy can be expressed, using residual properties, as
H 2 2 H 1 5 sH 2 2 H ig2 d 1 sH ig2 2 H ig1 d 2 sH 1 2 H ig1 d
(6.125)
In this expression:
H 2 and H 1 represent the molar enthalpy of a substance at two different states.
H 2ig represents the molar enthalpy of a hypothetical ideal gas at state 2.
PitfaLL
PreventiOn
When using residual
properties, it is easy
to accidentally flip
the sign of one of the
terms. One way to
avoid sign errors is to
verify that the ideal
gas properties cancel
out, as they do on the
right-hand side of
Equation 6.125.
H 1ig represents the molar enthalpy of a hypothetical ideal gas at state 1.
An analogous expression can be written for any other intensive state property.
We can find the change in molar enthalpy for any process involving a pure compound using Equation 6.125. Practically speaking, Equation 6.125 is helpful when
the heat capacity is unknown at the specific conditions of interest but the ideal gas
heat capacity C P* is available, because the expression H 2ig – H 1ig can be evaluated
using C P* . Consider the application of Equation 6.125 to Example 6-9, which is also
illustrated in Figure 6-12.
Define states 1 and 2 to represent the gas entering the turbine (1) and leaving the
turbine (2).
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
P1, T1
Actual path
P2, T2
DH 5 H2 2 H1
Isothermal
pressure change
DH 5 H1ig 2 H1
P5 0
T 5 T1
271
Isothermal
pressure change
DH 5 H2 2 H2ig
Temperature
P5 0
T 5 T2
change, ideal gas
DH 5 H2ig 2 H1ig
Figure 6‑12 Summary of method for calculating
change in enthalpy using the residual enthalpy.
H 2 2 H 1 represents the change of molar enthalpy for the gas as it travels through
the turbine.
H 2ig 2 H 1ig represents the change in molar enthalpy for a hypothetical ideal gas
experiencing the same change in temperature and pressure as the real gas.
H 2 2 H 2ig represents the residual molar enthalpy at state 2: How much the gas
leaving the turbine departs from ideal gas behavior. Analogously, H 1 2 H 1ig represents the residual molar enthalpy for the gas entering the turbine.
Notice that in Equation 6.125 the left and right-hand sides both equal H 2 2 H 1,
as the properties of the hypothetical ideal gas cancel out on the right-hand side.
A comparable approach can be applied to molar entropy:
S 2 2 S 1 5 sS 2 2 S ig2 d 1 sS ig2 2 S ig1 d 2 sS 1 2 S ig1 d
(6.126)
And analogous equations can be written for molar internal energy, molar Gibbs free
energy, etc. In order to apply such an approach to solving problems, we must have a
way to relate residual property expressions like H 2 H ig or U 2 U ig to measurable
properties.
6.3.4 Mathematical expressions for residual Properties
How can we put a quantitative value on a residual property at a particular temperature and pressure? Recall that all gases approach ideal gas behavior as pressure
decreases. In the limiting case of P = 0, all gases will be ideal gases. This in turn
means that at this state the real gas properties are identical to the ideal gas properties; at P = 0, all residual properties will be 0.
We can use the P = 0 condition as a starting point in computing a residual property. For example, consider the state T = 600 K and P = 10 bar found in Example 6-9.
The value of the residual molar enthalpy is unknown at this T and P, but we know it
is 0 at T = 600 K and P = 0.
Talking about a gas at
P = 0 (e.g., methane
at P = 0) may seem
unrealistic, because for
the pressure in a container to be literally 0,
there would be no
molecules. Mathematically, however, there
is nothing wrong with
modeling a gas at the
limiting condition of
P = 0 bar.
H 2 H ig 5 0 at T 5 600 K, P 5 0
We also know that the ideal gas molar enthalpy is independent of pressure. Thus, as
we isothermally increase the pressure, H changes, but H ig remains constant. Consequently, the residual molar enthalpy is equal to the change in molar enthalpy for the
gas as it is isothermally compressed from P = 0 bar to P = 10 bar. Thus,
sH 2 H igdT5600 K, P510 bar 5
#
T5600 K, P510 bar
dH
(6.127)
T5600 K, P50 bar
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272 Fundamentals of Chemical Engineering Thermodynamics
In Example 6-9, we explored the relationship between molar enthalpy and temperature and pressure and arrived at Equation 6.123:
3
dH 5 CP dT 1 V 2 T
Equation 6.128 is
valid for any
isothermal process.
1 −T 2 4 dP
−V
P
In assessing residual properties, we are considering a change in pressure while holding temperature constant, so Equation 6.123 simplifies to
3
dH 5 V 2 T
1 −T 2 4 dP
−V
(6.128)
P
Combining Equations 6.127 and 6.128 gives
sH 2 H igdT5600 K, P510 bar 5
#
T5600 K, P510 bar
T5600 K, P50 bar
3V2T 1 −T 2 4 dP
−V
(6.129)
P
Equation 6.129 can be integrated if an equation of state describing the gas
is available. While Equation 6.129 is specific to the condition T = 600 K and
P = 10 bar, the logic behind the derivation is applicable to any temperature and
pressure. Thus,
H 2 H ig 5
#
T5T, P5P
T5T, P50
3V 2 T 1 −T 2 4 dP
−V
(6.130)
P
It is significant to note that the integral in Equation 6.130 is carried out along an
isothermal path. This is a common feature of all residual properties; in evaluating
this integral, T can be treated as a constant.
The residual molar entropy is slightly more complicated to derive, because S is
a function of both temperature and pressure—even for an ideal gas. It is, however,
still true that the residual is 0 at any temperature (T) when P = 0. Thus, the residual
molar entropy at a higher pressure can be found by quantifying the changes in both
S and S ig as pressure increases.
S 2 S ig 5
#
T5T, P5P
dS 2
T5T, P50
#
T5T, P5P
dS ig
(6.131)
T5T, P50
Example 6-7 explored the relationship among S, T, and P, leading to Equation 6.106.
Thus,
1 −T 2 dP 1 1−T2 dT
dS 5 2
−V
−S
P
As for enthalpy, the
residual entropy is
calculated on an
isothermal path. The
path begins at P = 0,
because real and ideal
gas properties are
identical at that point,
but there is no reason
to vary from the actual
temperature.
P
which, for an isothermal process, simplifies to
1 −T 2 dP
dS 5 2
−V
(6.132)
P
Because the integrals in Equation 6.131 are carried out on a constant-temperature
path, we can combine Equations 6.131 and 6.132 for
S 2 S ig 5
#
T5T, P5P
T5T, P50
2
1 −T 2
−V
dP 2
#
T5T, P5P
T5T, P50
P
2
1 −T 2 dP
−V ig
(6.133)
P
The first integral requires an equation of state that describes the real gas. The second,
however, can be evaluated further. For an ideal gas,
V ig 5
RT
P
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
273
and
1 −T 2 5 RP
−V ig
P
This result can be inserted into Equation 6.133. Since both integrals are carried out
with respect to dP and between the same limits, they can be combined as
#
S 2 S ig 5 2
T5T, P5P
T5T, P50
31 −T 2 2 RP4 dP
−V
(6.134)
P
The residual molar internal energy could be derived in an analogous manner, but it
is instructive instead to recall the relationship between U and H. By definition,
H 5 U 1 PV
which means that
H 2 H ig 5 sU 1 PV d 2 sU ig 1 PV igd
(6.135)
Rearranging, and recognizing that for an ideal gas PV = RT:
H 2 H ig 5 sU 2 U igd 1 sPV 2 RT d
(6.136)
Solving for U – U ig and also inserting the definition that Z = PV/RT gives
The compressibility
Z was first defined in
Section 2.3.4.
U 2 U ig 5 RTs1 2 Zd 1 sH 2 H igd
U 2 U ig 5 RT s1 2 Zd 1
#
T5T, P5P
T5T, P50
3V 2 T 1 −T 2 4 dP
−V
(6.137)
P
Alternative expressions of the residual properties can be derived by noting that,
when P = 0, the molar volume is infinite. Thus, V = ∞, like P = 0, is a condition at
which all gases will show ideal gas behavior, and all residual properties are 0. When
the integral is carried out with respect to dV, the residual molar enthalpy becomes
H 2 H ig 5 RT sZ 2 1d 1
#
T5T, V5V
T5T, V5`
2 P4 dV
3T 1−P
−T 2
V
(6.138)
Equations 6.130 and 6.138 are two different expressions for the residual molar enthalpy. They are both completely general, so it is never “right” or “wrong” to use
either; we simply use whichever is more convenient for the case at hand. Note that
Equation 6.130 involves a partial derivative of V and Equation 6.138 includes a partial derivative of P, whichever you use is guided by whether the equation of state
being employed is more easily solved for P or V.
6.3.5 summary of residual Properties
We close this section by summarizing the residual property expressions for molar
internal energy, enthalpy, and entropy. We also, at this point, introduce commonly
used shorthand: symbols of the form MR are often used to represent residual properties of the form M – M ig.
uSage
The change in any intensive property M between states 1 and 2 can be quantified
using residual properties as
M 2 2 M 1 5 ( M 2 2 M ig2 ) 1 ( M ig2 2 M ig1 ) 2 ( M 1 2 M ig1 )
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274 Fundamentals of Chemical Engineering Thermodynamics
or
M 2 2 M 1 5 M R2 1 ( M ig2 2 M ig1 ) 2 M R1
(6.139)
inTegraTing WiTh reSPeCT To dP
U 2 U ig 5 UR 5 RTs1 2 Zd 1
#
T5T, P5P
T5T, P50
H 2 H ig 5 HR 5
3V 2 T 1 −T 2 4 dP
(6.140)
−V
P
3
1 −T 2 4 dP
(6.141)
T5T, P50
#
31 −T 2 2 RP4 dP
(6.142)
#
T5T, P5P
S 2 S ig 5 SR 5 2
T5T, P5P
T5T, P50
V2T
−V
P
−V
P
inTegraTing WiTh reSPeCT To dV
U 2 U ig 5 UR 5
#
T5T, V 5 V
T 5 T,V 5`
H 2 H ig 5 HR 5 RTsZ 2 1d 1
#
2 P4 dV
3T 1−P
−T 2
V
T5T, V5V
T5T, V5`
S 2 S ig 5 SR 5 R lnsZd 1
#
T5T, V5V
T5T, V5`
2 P4 dV
3T 1−P
−T 2
V
R
2 4 dV
31−P
−T 2
V
V
(6.143)
(6.144)
(6.145)
6.3.6 application of residual Properties
We begin this section by returning to Example 6-9, a turbine example posed in Section 6.3.2. It was used to illustrate the barriers that arise when CP is known for the
ideal gas case but not in general and, thus, motivate residual properties. Now that
we have mathematical expressions for the residual molar enthalpy, we use them to
complete the problem.
examPle 6‑9
The van der Waals
equation of state was
first introduced in
Section 2.3.4.
revisited: aPPLying residuaL PrOPerties
Methane enters a turbine at T = 600 K and P = 10 bar and leaves at T = 400 K and
P = 2 bar. How much work is produced for each mole of gas? Use the following data: van
der Waals constants for methane are a = 0.2303 Pa-m6/mol2 and b = 4.306 3 1025 m3/mol,
and CP* for methane is given in Appendix D.
SOLUTION:
Step 1 Define a system and write an energy balance
Our previous inspection of this turbine process resulted in the energy balance:
·
WS
5 H2 2 H1
n·
(6.146)
Previously we looked at using a total derivative as a starting point for finding the change
in molar enthalpy, and in the process established a motivation for residual properties.
Now we will solve the problem using the residual molar enthalpy.
Step 2 Apply residual molar enthalpy
Equation 6.139 can always be used as a starting point when applying residual properties.
For enthalpy we have
H 2 2 H 1 5 H R2 1 ( H ig2 2 H ig1 ) 2 H R1
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
275
Step 3 Find the ideal gas change in molar enthalpy
Section 2.3.3 stated that for an ideal gas, dH = CP* dT, and gave a physical rationalization
for this equality. The equality will now be demonstrated mathematically. We can begin by
writing the total derivative for enthalpy in terms of T and P for
dH 5
1 −T 2 dT 1 1 −P 2 dP
−H
−H
P
(6.147)
T
In Section 6-2, we derived Equation 6.64:
1 −P 2 5 V 2 T 1 −T 2
−H
−V
T
P
This, along with the definition for CP, can be introduced into Equation 6.147:
3
dH 5 CP dT 1 V 2 T
1 −T 2 4 dP
−V
(6.148)
P
For an ideal gas, (∂V /∂T)P is determined by
V5
RT
P
1 −T 2 5 RP
−V
(6.149)
P
Introducing this into Equation 6.148 gives
3
dH 5 CP dT 1 V 2 T
1RP24 dP 5 C dT 1 0 dP
P
(6.150)
dH 5 CP dT
Consequently, for an ideal gas, dH = CP* dT, even if the pressure is not constant.
According to Appendix D, the ideal gas heat capacity of methane is
CP*
R
5 4.568 2 s8.975 3 1023dT 1 s3.631 3 1025dT 2
Here we’ve proved
mathematically what
was stated in Section 2.3.3; that molar
enthalpy is only a
function of temperature for an ideal gas.
(6.151)
2 s3.407 3 1028dT 3 1 s1.091 3 10211dT 4
Thus we know that if methane was an ideal gas at the conditions in this example, the
change in molar enthalpy would be
1
H ig2 2 H ig1 5 8.314
J
mol ? K
2#
T5400 K
[4.568 2 s8.975 3 1023dT
(6.152)
T5600 K
1 s3.631 3 1025dT 2 2 s3.407 3 1028dT 3
1 s1.091 3 10211dT 4] dT
H ig2 2 H ig1 5 37.98T 2 s0.07462d
T2
T3
T4
1 s3.019 3 1024d
2 s2.833 3 1027d
2
3
4
*
T5400 K
1 s3.019 3 1024d
H ig2 2 H ig1 5 14,035
T5
5 T5600 K
J
J
J
2 23,324
5 29289
mol
mol
mol
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276 Fundamentals of Chemical Engineering Thermodynamics
Step 4 Relate the residual molar enthalpy to temperature and pressure
Equations 6.130 and 6.138 give expressions for the residual molar enthalpy. These are
both completely general equations; we are free to use whichever one is convenient for
the case at hand. Since the van der Waals equation gives P explicitly, and cannot be readily solved for V, it is much easier to evaluate (∂P/∂T)V than (∂V/∂T)P, so we will apply
Equation 6.138
HR 5 RT sZ 2 1d 1
#
T5T, V5V
T5T, V5`
3T 1−T2 2 P4 dV
−P
V
The integral is with respect to dV, so we need to relate everything to V in order to evaluate the integral. The van der Waals equation of state is
P5
RT
a
2
V 2 b V2
(6.153)
Taking the derivative of both sides with respect to T at constant V gives
The van der Waals
equation of state was
first introduced in
Section 2.3.4.
R
5
1−P
2
−T
V2b
(6.154)
V
Because V is being held constant, the a/V 2 term is a constant, and its derivative is zero.
Substituting these expressions for P and (∂P/∂T)V into the integral gives
HR 5 RTsZ 2 1d 1
#
T5T, V5V
T5T, V5`
3T 1V 2 b2 2 1V 2 b 2 V 24 dV
HR 5 RTsZ 2 1d 1
R
RT
a
2
#
T5T, V5V
T5T, V5`
3V 4 dV
a
2
(6.155)
which integrates to
It is not a coincidence
that the integral is 0
at the lower limit of
integration, since this
lower limit (V = ∞)
represents the exact
condition at which
real gases behave like
ideal gases.
If the van der Waals
equation is written as
a polynominal in V,
the highest order
term is V 3. Thus it is
‘cubic’ in V.
HR 5 RT sZ 2 1d 1
2a
V
*
V5V
V5`
(6.156)
Since –a / V is 0 at V = ∞, the final expression is
HR 5 RTsZ 2 1d 2
a
V
(6.157)
Step 5 Determine V 1 and V 2
Equation 6.157 is an expression for the residual molar enthalpy for substances that are
described by the van der Waals equation of state, valid at any set of conditions. To find a
numerical value for this residual at the conditions of interest (P1, T1 and P2, T2), we need
numerical values for V 1 and V 2. Conceptually, when we know two variables among P, V,
and T and we need the third, we should always look to apply an equation of state. In this
case, finding a numerical value for V is somewhat complicated by the fact that the van
der Waals equation of state is cubic in V. Appendix B reviews methods of solving a cubic
equation. In this case, the solutions are
For P1 = 10 bar and T1 = 600 K, V 1 = 4.986 3 10–3 m3/mol
For P2 = 2 bar and T2 = 400 K, V 2 = 0.01659 m3/mol
Cubic equations have three roots, but in this case, the other two solutions for the molar
volume are complex numbers, which are physically meaningless solutions. Section 7.2.3
discusses the situation in which an equation of state has more than one real-number solution for volume.
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
277
The compressibility factor Z can now be found for both the inlet and outlet.
Z1 5
P1 V1
RT1
Z2 5
1
s10 bard 4.986 3 1023
5
P2 V 2
RT2
1
21
100 cm
1m
2
bar ? cm3
s600 Kd
83.14
mol ? K
1
2
3
5 0.9995
2 1 1m 2
5 0.998
bar ? cm
183.14 mol ? K 2 s400 Kd
s2 bard 0.01659
5
m3
mol
(6.158)
100 cm 3
m3
mol
3
(6.159)
Applying Equation 6.157 to the conditions of the entering and exiting gas, we have
a
H R1 5 RT sZ1 2 1d 2
V1
1
H R1 5 8.314
2
J
s600 Kds0.9995 2 1d 2
mol ? K
1
Pa ? m6
mol2
m3
4.986 3 1023
mol
H R1 5 248.7
J
mol
H R2 5 220.5
J
mol
0.2303
2
1J
Pa ? m3
(6.160)
and analogously,
The calculated values
of Z are close to 1,
which suggests the
methane closely approximates ideal gas
behavior. Thus it is
logical that the calculated residuals are
relatively small.
Step 6 Combine calculated results into expression from step 2
From Equation 6.139,
H 2 2 H 1 5 H R2 1 ( H ig2 2 H ig1 ) 2 H R1
The terms on the right-hand side have now all been computed for
H 2 2 H 1 5 s220.5d
J
J
J
J
1 s29289d
2 s248.7d
5 29261
mol
mol
mol
mol
(6.161)
The energy balance revealed that this change in molar enthalpy is identical to the work
produced per mole, which is what we are trying to determine.
The problem-solving strategy established in Example 6-9, using the residual molar
enthalpy, can be applied analogously to other intensive properties. Example 6-10
uses residuals to determine change in internal energy, and Example 6-11 applies the
residual molar entropy.
PistOn-cyLinDer eXaMPLe using resiDuaLs
examPle 6‑10
One mole of gas is confined in a piston-cylinder apparatus (Figure 6-13) initially at
T 5 300 K and P 5 10 bar. The gas is heated reversibly and at constant pressure to
T 5 500 K. What are the values for Q and W as a result of this process?
The ideal gas heat capacity for this gas is constant at CV*=3R, and the van der Waals
parameters for the gas are a = 0.250 Pa-m6/mol2 and b = 3.00 3 1025 m3/mol.
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278 Fundamentals of Chemical Engineering Thermodynamics
In previous examples,
the ideal gas constant
pressure heat capacity
CP* was given; here the
constant-volume heat
capacity CV* is known.
Recall that, for an
ideal gas, these can be
related to each other,
CV* + R = CP*
P2 5 10 bar
T2 5 500 K
Reversible
heating
P1 5 10 bar
T1 5 300 K
Figure 6‑13 Piston-cylinder system described in
Example 6-10.
SOLUTION:
Step 1 Define a system and write an energy balance
The gas inside the piston-cylinder device is the system. The work done by an expanding
system is something we have calculated several times (first in Example 1-4), and it can
be determined without an energy balance. But as usual, we need an energy balance to
find Q, so we will start there. This is an unsteady-state process and also a closed system.
Since the cylinder itself is stationary, we can neglect changes in potential and kinetic
energy. The energy balance, therefore, is (see also Example 3-10 for a more detailed
derivation)
U 2 2 U 1 5 Q 1 WEC
(6.162)
Molar internal energy is a state function, and because we know two intensive properties
(T, P) for the initial and final states, we should be able to compute U 2 – U 1.
PitFaLL
PreventiOn
A common conceptual
error is to say, “The
(∂P/∂T)V term must
be 0, because this is
a constant-pressure
process. However, a
residual is a state property; its value is not
path dependent.
A more detailed
discussion of this conceptual error is given
at the end of the problem solution.
Step 2 Relate U2 2 U1 to residual properties
The ideal gas heat capacity is known, but the pressure is 10 bar. While the methane in
Example 6-9 had Z~1 at P 5 10 bar, many gases would not be well modeled as ideal at
this pressure. Consequently we will not assume ideal gas behavior; we will calculate the
residual properties and see whether they are significant or not.
To find the change in molar internal energy using residual properties, we begin by
writing an equation for U that is analogous to Equation 6.139:
U 2 2 U 1 5 U R2 1 ( U ig2 2 U ig1 ) 2 UR1
(6.163)
Step 3 Evaluate ideal gas term of Equation 6.162
The middle term of the right-hand side of Equation 6.163 is familiar: As first discussed in
Section 2.3.3, internal energy for an ideal gas is a function of temperature only:
( U ig2 2 U ig1 ) 5 CV* sT2 2 T1d 5 3R sT2 2 T1d
(6.164)
Step 4 Evaluate residual molar internal energy for this equation of state
Next we need to evaluate the residuals at the initial and final states. We can start
with Equation 6.143, which gives the residual molar internal energy in its general
form:
U 2 U ig 5 U R 5
#
V5V
V5`
2 P4 dV
3T 1−P
−T 2
V
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
279
We need to express the term in brackets as a function of V, so that we can integrate it
with respect to dV. The van der Waals equation of state is
RT
a
2 2
V2b V
P5
(6.165)
Taking the derivative of both sides with respect to T at constant V gives
1−T2 5 V R2 b
−P
(6.166)
V
Notice that because V is being held constant, the a/V 2 term is a constant, and its derivative is zero.
Substituting these expressions for P and (∂P/∂T)V into the integral gives
UR 5
#
V5V
V5`
3T 1V 2 b2 2 1V 2 b 2 V 24 dV
R
RT
a
2
(6.167)
The first two terms within the integral cancel, leaving
#
UR 5
V5V
V5`
3Va 4 dV
2
The integral of a/V 2 is – a/V. Since – a/V is zero at the lower limit of integration (V = ∞),
the final result is
UR 5
2a
V
(6.168)
Step 5 Find numerical values for molar volumes V 1 and V 2
The only thing we have assumed up to this point is that the gas follows the van der Waals
equation of state, so the expression for the internal energy residual function is valid at
any temperature and pressure. We need to evaluate – a/V at P1, T1 and at P2, T2. Solving
the van der Waals equation for V gives
At P1 = 10 bar and T1 = 300 K, V 1 = .00242 m3/mol
At P2 = 10 bar and T2 = 500 K, V 2 = 0.00413 m3/mol
The van der Waals
equation is cubic in V;
it has three solutions
for V. Section 7.2.4
discusses the case in
which more than one
of the solutions
are real.
As in Example 6-9, there is only one real solution for the cubic equation in this case; the
other two solutions for V are complex numbers.
Step 6 Solve for change in molar volume
Recall that Equation 6.139 stated:
U 2 2 U 1 5 U R2 1 ( U ig2 2 U ig1 ) 2 U R1
We can now express all three terms as functions of measurable properties as
U 2 2 U1 5
2a
1 3R sT 2 T d 2 1 2
12a
V 2
V
2
2
(6.169)
1
m6
m6
0.250 Pa
2
mol
J
mol2
1
3
8.314
s500
K
2
300
Kd
1
m3
mol K
m3
.00413
.00242
mol
mol
20.250 Pa
U 2 2 U1 5
1
1
2
The first and last term have units Pa · m3/mol, but a Pascal is a N/m2, so this is really a
N · m/mol, or equivalently J/mol. Thus, all three terms are in the same units and can be
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280 Fundamentals of Chemical Engineering Thermodynamics
summed. Since a and b were given with three significant figures, we will round our answer
(5031.7 J/mol) to three as well:
U 2 2 U 1 5 5030
J
mol
Note that if we had assumed this was an ideal gas and used U 2 2 U 1 5 CV (T2 2 T1),
the result would have been U 2 2 U 1 5 4990 J/mol. The residual functions are small in
magnitude, but not insignificant, compared to the ideal gas change in internal energy:
UR1 5 103.2 J/mol and UR2 5 260.6 J/mol. Because they are opposite in sign, significant
cancellation occurs.
Step 7 Calculate work
The work done on, or by, an expanding system was first derived in Example 1-4 and is
given by
WEC 5 2
# P dV
V2
(6.170)
V1
where P is the pressure opposing the motion. In this case, because the process is reversible, the pressure of the gas and the external pressure are essentially the same, and so
P can be considered constant at 10 bar. Integrating gives
1
WEC 5 2s10 bard 0.00413 2 .00242
m3
mol
21
WEC 5 21710
100 kPa
1 bar
21
N
m2
1 kPa
1000
21
1J
N?m
2
J
mol
Step 8 Close energy balance to find heat
Returning to the energy balance, we have
U 2 2 U 1 5 Q 1 WEC
5030
J
J
5 Q 2 1710
mol
mol
Q 5 6740
Just as H, U, and S are
state properties; the
residuals are also state
properties.
(6.171)
J
mol
PITFALL PREVENTION: A closer look at a common conceptual error
Let us return to the possible conceptual error mentioned in step 4. This is a constantpressure process, so why is the (∂P/∂T)V term not equal to zero when we are evaluating
the residual? Aren’t all derivatives of P equal to 0 if P is constant?
Conceptually, the thing to remember is that internal energy is a state function, and
the residual internal energy is also a state function. If the initial pressure was 5 atm and
the final pressure was 10 atm, the process would not be at constant pressure, and you
would not be tempted to cross out the pressure derivative. But because a residual is a
state function, it has a particular value at P = 10 bar and T = 300 K, which is path independent. In other words, the quantity (U 1 – U 1ig) depends only on P1 and T1; it does not
depend on P2 at all, so the fact that P2 and P1 happen to be equal to each other does not
affect the residual property. (∂P/∂T)V can only be evaluated using an applicable equation of state, which quantifies the relationship between the measurable state properties
P, V, and T.
Mathematically, a residual property integral is not carried out over the process path; it is
not integrated from P1,T1 to P2,T2. The residual compares a real substance to an ideal gas at a
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
281
particular T and P. It has P = 0 (or its equivalent of V = ∞) at the lower limit of integration,
and the actual P at the upper limit. Temperature is always a constant when we are evaluating
the integral to compute a residual property, but pressure is never constant.
Now, by contrast, suppose we had started the problem by writing a total derivative
expression for dU as
dU 5
1 −P 2 dP 1 1 −T 2 dT
−U
−U
T
(6.172)
P
This expression CAN be applied to the actual process path, so here it would be valid to
say, “This is a constant pressure process, so dP = 0.” This would, in fact, have been a logical start, but had we done this, we would have found (∂U/∂T)P is dependent upon CV. At
this point, we would have recognized that we know CV* but not CV , which would probably
have convinced us to use residual properties.
The final example is similar to the nozzle problem examined in Example 6-7, with the
only difference being what is stated about the heat capacity. Comparing the results
of the two examples, which use two distinct solution strategies, is instructive.
aPPLicatiOn OF enthaLPy and
entrOPy residuaL Func tiOns tO a nOzzLe
A gas flows through an adiabatic nozzle at a steady state, entering at P = 5 bar and T =
4008C and leaving at P = 1 bar. Assuming the nozzle is reversible, what is the temperature of the exiting gas? Assume the gas has an ideal gas heat capacity of CP* = 40 J/mol · K
and follows the equation of state:
RT
1 aTP
V5
P
where a = 1.00 cm3/mol · bar · K.
SOLUTION:
This problem is identical to Example 6-7 with one exception: The heat capacity was
treated as a constant CP = 40 J/mol · K in Example 6-7, but here this value is only considered valid at ideal gas conditions. Thus, the entropy balance in Example 6-7, which was
simply S in = S out, is again valid, but here we will use residual properties to relate molar
entropy to measurable properties.
examPle 6‑11
Residual functions
are used because CP
is given only for ideal
gas conditions, but the
pressure entering the
nozzle is P = 5 bar.
While some gases
may be well modeled
as ideal gases at this
pressure, we do not
automatically assume
that all gases are ideal
at this pressure.
Step 1 Relate S in 2 S out to temperature and pressure
To calculate change in molar entropy from residual properties, start with Equation 6.139 as
S out 2 S in 5 S Rout 1 ( S igout 2 S igin ) 2 S Rin
(6.173)
From the entropy balance, we know the left-hand side is 0:
0 5 ( S out 2 S igout ) 1 ( S igout 2 S igin ) 2 ( S in 2 S igin )
(6.174)
The solution strategy is to express the right-hand side in terms of known quantities and
the unknown temperature Tout.
Step 2 Evaluate ideal gas term
Section 4.4.1 examined the change in entropy for an ideal gas process. Here, the ideal gas
heat capacity is constant, which means that Equation 4.58 can be applied for
S igout 2 S igin 5 CP* ln
1 T 2 1 R ln 1P 2
Tout
Pin
in
out
(6.175)
If CP* were not constant, we would apply
the more general
Equation 4.54 instead
of Equation 4.59.
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282 Fundamentals of Chemical Engineering Thermodynamics
The entire right-hand side of Equation 6.175 is known except Tout, which is what we are
trying to find.
Step 3 Derive an expression for the residual molar entropy
We can evaluate the residual molar entropy starting with either Equation 6.142 or Equation 6.145. Here, the equation of state is given in the form V = f(T, P), so it is straightforward to assess partial derivatives of V. Consequently we choose to begin with Equation
6.142 as
S 2 S ig 5 2
#
T5T, P5P
T5T, P50
31 −T 2 2 RP4 dP
−V
P
The partial derivative (∂V/∂T)P is evaluated by differentiating both sides of the equation
of state
1 −T 2 5 RP 1 aP
−V
(6.176)
P
Plugging this result into Equation 6.142 gives
S 2 S ig 5 2
#
T5T, P5P
T5T, P50
S 2 S ig 5 2
#
31 P 1 aP2 2 RP4 dP
R
T5T, P5P
[aP] dP
T5T, P50
S 2 S ig 5
It would be reasonable, rather than
computing a residual
property for the outlet
gas, to have assumed
it’s negligible, since
the pressure is P =
1 bar. This calculation
reveals it to be very
small compared to the
residual at P = 5 bar.
2aP 2
2
(6.177)
Step 4 Apply results of steps 2 and 3 to Equation 6.174
Combining Equations 6.175 and 6.177 into Equation 6.174 gives
05
1 2 2
2aP 2out
1 CP* ln
1T 2
Tout
1 R ln
in
1P 2 1 2 2
Pin
2
2aP 2in
(6.178)
out
Inserting known values into Equation 6.178 gives
11 mol ?cmbar ? K2 s1 bard
3
2
052
(6.179)
2
1 mol ? K2
1 40
J
1
bar ? cm3
mol ? K
J
8.314
mol ? K
83.14
2
ln
1673.15 K2
Tout
11 mol ?cmbar ? K2s5 bard
3
2
1
bar ? cm
1 83.14
mol ? K
3
2 1 2
5 bar
ln
1
1 bar
2
Tout is the only unknown in the equation, and can be solved Tout 5 467.5 K 5 194.48C.
It is instructive to compare the results of Examples 6-7 and 6-11. In Example 6-7
we modeled the gas as having a constant CP = 40 J/mol · K, with the assumption
that this value was valid at ALL conditions. In Example 6-11, we used the same
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
numerical value for CP*, but only assumed it was valid at ideal gas conditions. Why
are the answers obtained from both examples identical? An explanation can be
obtained from Equation 6.124, which provides a relationship between heat capacity and pressure:
1 −P 2
−CP
5 2T
T
1−T 2
−2V
2
P
We determined previously (Equation 6.176) that for this particular equation of state:
1 −T 2 5 RP 1 aP
−V
P
There is no temperature dependence on the right-hand side, which means that when
we take the second derivative with respect to T:
1 −P 2
−CP
T
5 2T
1−T 2 5 0
−2V
2
(6.180)
P
This result reveals that, for this equation of state, heat capacity does not change with
pressure; the distinction between “ideal gas heat capacity” and “actual heat capacity” does not matter here.
Another point is that, while residual properties had not yet been introduced in
Section 6.2.6, there would have been nothing wrong with using them in Example 6-7.
If the heat capacity can be modeled as constant at ALL conditions, it must certainly
be constant at ideal gas conditions.
In sum, it makes sense that the two approaches taken in Example 6-7 (which
started with a total derivative) and Example 6-11 (which started with residual
properties) would produce the same answer; both are physically and mathematically
valid approaches. The answer is only accurate to the degree that the assumptions are
accurate: that V = RT/P + aTP and that CP = 40 J/mol · K. This equation of state was
contrived for illustrative purposes, and is not used in practice. Chapter 7 examines
the equations of state that are in common use by chemical engineers.
6.4 s u M M a r y O F c h a P t e r s i x
A total derivative can be written to relate changes in any intensive property to
changes in any two other intensive properties.
A common first step in solving problems is writing a total derivative that relates
the unknown property to the two intensive properties about which the most is
known.
Total derivative expressions are most readily solved when expressed in terms of
measurable properties: P, T, V, and their derivatives, CP and CV.
Among the useful tools for manipulating partial derivative expressions are the
following, which are presented mathematically in Section 6.2.7.
Fundamental property relations, which express U, H, G, and A as functions
of S, V, T, and P.
The expansion rule, which allows one to introduce two new variables into a
problem, and is often used in combination with fundamental property relations.
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283
284 Fundamentals of Chemical Engineering Thermodynamics
The triple product rule, which can be used to relate a partial derivative to
other partial derivates involving the same three variables.
Maxwell’s equations, which can often be used to convert a partial derivative
involving S into a partial derivative that involves only measurable properties.
The mathematical definitions of constant pressure heat capacity, constant
volume heat capacity, isothermal compressibility, and coefficient of thermal
expansion.
Equations of state can be used to resolve partial derivative expressions involving pressure, temperature and molar volume into solvable ODEs.
Residual properties are useful for gas phase processes in which an ideal gas heat
capacity is known, but the ideal gas model isn’t a reasonable approximation.
Residual properties can be related to measurable properties through a known
equation of state.
6.5 e x e r c i s e s
6-1.
Derive Equations 6.47 and 6.48.
6-2.
Derive Equations 6.61, 6.62, and 6.63.
6-3.
Find expressions for the isothermal compressibility
and coefficient of thermal expansion for an ideal
gas. Expressions should be in terms of measurable
properties only.
6-4.
Find expressions for the isothermal compressibility
and the coefficient of thermal expansion for a fluid
that is described by the van der Waals equation of
state. Expressions should be in terms of measurable
properties only.
6-5.
Find expressions for the residual molar internal energy, residual molar enthalpy, and residual molar
entropy for a fluid that is described by each of the
following:
A. The ideal gas law
B. The equation of state P = RT / V + a / V 2, in which
a is a constant.
C. The equation of state Z = 1 + (CP2)/(RT), in
which C is a constant.
RT
D. The equation of state V 5
1 aTP 2, in which
P
a is a constant.
6-6.
Equations 6.157 and 6.168 give the expressions for
HR and UR for a fluid described by the van der Waals
equation of state. For a compound that has a = 1.8 3
107 cm6bar/mol2 and b = 125 cm3/mol, compute HR
and UR at each of the following conditions. If there
is more than one value for V, find the HR and UR that
correspond to each value.
A. T = 400 K and P = 1 bar
B. T = 400 K and P = 5 bar
C. T = 600 K and P = 3 bar
D. T= 800 K and P = 10 bar
E. T = 2008C and P = 2 bar
F. T = 7008F and P = 25 psia
6.6 P r O b L e M s
6-7.
Demonstrate that if a gas follows the ideal gas law,
(∂U/∂P)T and (∂U/∂V )T are equal to 0.
6-8.
Derive an expression for each of the following that
is, as much as possible, in terms of measurable properties: P, V, T, CP, and CV, and their partial derivatives with respect to each other. S can appear in the
expression, but not derivatives of S.
A. (∂H/∂T)V
B. (∂H/∂P)T
C. (∂U/∂V)P
D. (∂U/∂T)P
E. (∂A/∂S)P
F. (∂A/∂S)T
G. (∂G/∂T)P
H. (∂G/∂P)S
6-9.
Prove that Equation 6.124 is valid. Hint: Start with
−H
−H
and
and use the fact that, in a mixed
P
−T
−P T
second partial derivative, the order of differentiation doesn’t matter.
1 2
1 2
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
285
6-10. Example 6-4 examined a valve and a liquid that had
a coefficient of thermal expansion V = 0.001°R21,
an isothermal compressibility of 0, and a molar volume V = 0.100 L/mol at T = 808F and P = 5 atm.
A. Estimate the molar volume of the liquid leaving
the valve, which has P = 0.5 atm and T = 80.28F.
B. Estimate the temperature at which the liquid
would have a molar volume V = 0.101 L/mol.
6-15. A. Five moles of an ideal gas is confined in a pistoncylinder device, initially at P1 = 1 bar and T1 =
400 K. First, the gas is compressed adiabatically
to P2 = 10 bar and temperature T2. Next the
piston is locked in place, and the gas is cooled
isochorically to T3 = 400 K and pressure P3. Determine Q, W, and ΔU for the gas in each of the
two steps of this process. Also find T2 and P3.
6-11. This problem is an extension of Problem 2-19. Two
moles of a gas are confined in a piston-cylinder. Initially,
the temperature is 300 K and the pressure is 1 bar. The
gas is compressed isothermally to 5 bar. The gas has
CP* = (7/2)R. Find Q, W, ∆U, ∆H, and ∆S for the gas if
A. The gas is an ideal gas.
B. The gas follows the van der Waals equation of
state with a = 5.0 3 106 bar ? cm6/mol2 and b =
30 cm3/mol.
B. Five moles of a gas that is described by the
equation of state given above is confined in a
piston-cylinder device. Calculate ∆U when this
gas undergoes the same two changes in state described in part A, from T1, P1 to T2, P2 and from
T2, P2 to T3, P3.
6-12. One mole of a gas is placed in a closed system with a
20 L vessel initially at T = 300 K. The vessel is then
isothermally expanded to 40 L. The gas follows the
equation of state:
P = RT/V + a/V 2
2
where a = 240 L · atm/mol2 and R = 0.08206 L · atm/
mol · K.
A. Derive an expression relating (dH/dV)T to measurable properties.
B. Find DH for the gas in this process.
6-13. A gas has an ideal gas heat capacity of CP* = (7/2)R
and is described by the equation of state:
Z = 1 + (CP 2)/(RT)
with C = 100 cm3/bar · mol.
A. Find a general expression for the residual molar
enthalpy for this gas.
B. Find a general expression for the residual molar
entropy for this gas.
C. Find DH and DS for the gas if it is isothermally
compressed from P = 1 bar and T = 400 K to
P = 50 bar and T = 400 K.
Problems 14 through 17 involve a gas that follows the
equation of state:
V5
RT
1 aTP 2
P
with a = 0.3 cm3/mol · bar 2 · K.
The gas has a molecular mass of 120 g/mol and an ideal gas
heat capacity CP* = 40 J/mol.
6-14. A gas, which follows the EOS given above, enters
a turbine at P = 15 bar and T = 500 K, and leaves
with a pressure of P = 0.5 bar. If the turbine has an
efficiency of 80%, find
A. The work produced in the turbine.
B. The actual temperature of the exiting stream.
For both parts A and B, assume the gas has CP* =
40 J/mol · K.
6-16. A gas with a flow rate of 300 mol/min enters a
steady-state, adiabatic nozzle with negligible velocity at T = 500 K and P = 10 bar and leaves the
nozzle at P = 1 bar. The gas has CP* = 40 J/mol · K.
A. Find the exiting temperature and exiting velocity
if the gas is ideal and the nozzle is reversible.
B. Find the exiting temperature and exiting velocity
if the gas follows the equation of state above and
the nozzle is reversible.
C. The gas follows the equation of state above, but
the nozzle is not reversible; it has a rate of en·
tropy generation of Sgen = 1.5 kJ/min. Find the
exiting temperature and exiting velocity.
6-17. A gas stream with a flow rate of 1000 mol/min leaves
a reactor at T = 450 K and P = 10 bar. For the next
step, the temperature of the gas must be reduced
to 300 K, but the pressure of the stream isn’t very
important. Consequently, a turbine is being placed
after the reactor, to obtain some work, but the turbine must be designed to have an actual outlet temperature of 300 K. Find the turbine outlet pressure
and the work produced for each of the following
cases. In all cases, the gas has CP* = 40 J/mol · K.
A. The gas is an ideal gas, and the turbine is
reversible.
B. The gas is an ideal gas, but the turbine efficiency
is 80%. Find the turbine outlet pressure and the
work produced.
C. The turbine is reversible, and the gas follows the
equation of state.
D. The turbine efficiency is 80%, and the gas follows
the equation of state.
6-18. One mole of a gas is compressed at a constant temperature of 400 K from P = 1 bar to P = 5 bar. The
gas is known to follow the equation of state:
V = RT/P + aP2
where a = 0.01 L/bar2 · mol.
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286 Fundamentals of Chemical Engineering Thermodynamics
The ideal gas heat capacity at T = 400 K is CP*= 3.85R.
A. Demonstrate that the heat capacity of this gas is
independent of pressure.
B. Derive an expression for (dU/dP)T as a function
of measurable properties.
C. Find DU for the gas in this process.
D. Find DS for the gas in this process.
6-19. A liquid has constant coefficient of thermal expansion and isothermal compressibility, V = 2.37 3
1023 K21 and T = 3.54 3 1025 bar21. The molar volume of the compound at T = 300 K and P = 1 bar is
V = 0.100 L/mol, and its heat capacity can be modeled as constant at CV = CP = 27.5 J/mol · K.
This liquid is stored in tank that has a “floating
top.” Thus, when the temperature changes, causing
the liquid to expand or contract, the container expands or contracts. For each of the processes given,
the tank initially has V = 30 m3, T = 300 K, and
P = 1 bar.
A. The “floating top” works properly, maintaining
the tank at a constant pressure. 5000 kJ of heat
are added to the tank. Find the final temperature
and volume.
B. The “floating top” malfunctions, such that the
container is effectively rigid at V = 30 m3. 5000 kJ
of heat are added to the tank. Find the final temperature and pressure.
6-20. The liquid described in Problem 6-19 is compressed
in a steady-state, adiabatic pump. 5 mol/s enter the
pump at T = 300 K and P = 1 bar and exit the pump
at P = 20 bar.
A. Using the approximation that V is a constant at
0.100 mol/L, compute the rate at which work is
done in the pump.
B. Assuming the value of work determined in part A
is correct, find the temperature of the liquid leaving the pump. Use relevant physical properties
given in Problem 6-19.
C. Assuming the temperature found in part B is correct, find the molar volume of the liquid leaving
the pump. Use relevant physical properties given
in Problem 6-19.
D. We assumed in part A that V was constant, but
then in part C we computed the change in V for
this process. Do you consider the answers obtained in parts A–C valid? How could you modify the solution approach to get a more accurate
answer?
6-21. Using the compressed liquid tables, estimate the isothermal compressibility and coefficient of thermal
expansion for liquid water at each of the following
conditions.
A. P = 100 bar and T = 1008C
B. P = 200 bar and T = 1008C
C. P = 100 bar and T = 2008C
D. P = 200 bar and T = 2008C
E. Based upon the results of A–D, comment on how
valid it would be to model the isothermal compressibility and coefficient of thermal expansion
for liquids at high pressure as constants.
6-22. Re-do Problem 4-9. Instead of using the steam tables, assume that the material entering and leaving
the turbine is all vapor, and that steam is accurately
described by the van der Waals equation with a =
0.5542 Pa m6/mol2 and b = 3.051 3 1025 m3/mol.
Comment on the accuracy of the van der Waals
equation in this case.
6-23. This problem is a variation on Problem 4-16, in
which pressure was in the range 0.5 to 1 bar, and we
used the ideal gas model. Here we explore a similar
process with higher pressures and a real gas model.
Ten moles of gas are placed in a rigid container.
Initially the gas is at P = 5 bar and T = 300 K, and the
container is also at T = 300 K. The container is placed
in a furnace, where its surroundings are at a constant
T = 600 K. The container is left in the furnace until
both the container and the gas reach thermal equilibrium with the surroundings. The gas is described by
the van der Waals equation of state, with a = 8.0 3 105
cm6 · bar/mol2 and b = 100 cm3/mol, and has an ideal
gas heat capacity of CV* = 2.5R. The container itself has
a mass of 10 kg (not including the mass of the gas inside) and a heat capacity of ĈV = 1.5 J/g · K.
A. Find the heat added to the gas.
B. Find the heat added to the container.
C. Find the change in entropy of the gas.
D. Find the change in entropy of the universe.
E. What aspect of this process is irreversible?
6-24. Methane enters a process at T = 3008F and P = 1 atm,
and is heated and compressed to T = 4008F and
P = 5 atm. Find the change in molar Gibbs free energy for the methane, using Figure 7-1.
6-25. One mole of a gas is compressed at a constant temperature of 400 K from P = 0.1 bar to P = 10 bar.
The gas is known to follow the equation of state:
V = RT/P + aP2
where a = 0.025 L/bar2mol.
The ideal gas heat capacity at T = 400 K is CP* = 3.85 R.
A. Find an equation for the residual G for a gas that
follows this equation of state.
B. Find the change in Gibbs free energy for this
process.
6-26. Find the change in Gibbs free energy for the system
and process described in Problem 6-15, part B.
The Helmholtz energy (A), which was introduced in this
chapter, is rarely used in this introductory book, but has
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C H A P T E R 6 Thermodynamic Models of Real, Pure Compounds
great utility in the field of statistical thermodynamics, so
being able to model changes in A with respect to measurable properties is essential. The next three problems
examine changes in A.
287
6-29. A gas has CP* = 35 J/mol · K, and follows the equation of state:
6-27. Steam is heated from an initial condition of saturated steam at P = 1.5 bar to a final state of P =
3 bar and T = 3008C. Use the steam tables to find
the change in  for this process.
6-28. Find the change in Helmholtz energy for the process
described in Problem 6-18.
V5
RT
1 aTP2
P
with a = 0.15 cm3/mol · bar2 · K.
Find the change in A when the gas is compressed
isothermally from T = 300 K and P = 1 bar to
P = 10 bar.
6.7 g LO s s a r y O F s yM b O L s
A
Helmholtz energy
A
molar Helmholtz energy
a
parameter in van der Waals
EOS, accounting for intermolecular interactions
b
CP
CP*
ĈV
ĈP
parameter in van der Waals
EOS, accounting for
excluded volume
Gig
molar Gibbs free energy for
an ideal gas state
GR
residual molar Gibbs free
energy
H
enthalpy
Ĥ
H
specific enthalpy
residual molar entropy
T
temperature
U
internal energy
U
molar internal energy
Uig
molar internal energy for an
ideal gas state
residual molar internal
energy
molar enthalpy
UR
Hig
molar enthalpy for an ideal
gas state
V
volume
constant-pressure heat
capacity (molar basis) for
ideal gas
HR
residual molar enthalpy
V
molar volume
M
·
m
mass of system
Vig
molar volume for an ideal
gas state
constant-volume heat capacity (mass basis)
N
·
n
number of moles in system
W
work
molar flow rate
WEC
constant-pressure heat
capacity (mass basis)
P
pressure
work of expansion/
contraction
Q
heat
Z
compressibility factor
V
coefficient of thermal
expansion
T
isothermal compressibility
JT
Joule-Thomson coefficient
constant-pressure heat capacity (molar basis)
mass flow rate
constant-volume heat capacity (molar basis)
R
gas constant
CV*
constant-volume heat capacity (molar basis) for ideal gas
S
entropy
S
molar entropy
G
Gibbs free energy
Sig
G
molar Gibbs free energy
molar entropy for an ideal
gas state
CV
SR
6.8 r e F e r e n c e
Ben-Zion, M., U.S. Patent #5,522,870.
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7
Equations of State (EOS)
Learning Objectives
This chapter is intended to help you learn:
How a cubic equation of state can be used to model a compound in the liquid, vapor,
gas, and supercritical states
To solve equations of state, and correctly interpret the results when multiple solutions
exist
How to compute values for the adjustable parameters in a cubic equation of state
To recognize the analogies between different compounds that are revealed by the
principle of corresponding states, and how they can be used to estimate unknown
physical properties
Estimation of physical properties using group additivity
The basic elements of microscopic modeling
C
hapter 2 introduced the need for reliable models that provide estimates of physical and thermochemical properties, for situations in which experimental data is not
available. The models introduced at that time (e.g., ideal gas model) were simplistic,
and valid only under specific conditions.
Chapter 6 introduced a broader range of modeling techniques that are applicable to
any pure compound at any conditions, but using them often requires an equation of state.
In Chapter 2, we defined an “equation of state” (frequently abbreviated EOS) as a mathematical relationship among P, V , and T, such that if any two are specified, the third can be
calculated. So far, the only equations of state we have introduced are the ideal gas law and
van der Waals. The motivational examples take a closer look at these and demonstrate the
need for more versatile equations of state.
Chapter 6 contained
some additional equations of state that
were contrived by the
authors for illustrative
purposes but have
never been used in
engineering practice.
7.1 MOTIVATIONAL EXAMPLES: transportation of natural gas
Natural gas is a widely used fuel source, composed primarily of methane. Within the
continental United States, it is most commonly transported by pipeline. However,
when natural gas is shipped overseas, or to remote locations, an alternative to building more pipelines is transporting it by ship. This involves liquefying the natural gas,
so that it can be stored in small volumes for transport. Liquefied natural gas (LNG)
is hazardous because it is flammable and also because of the extreme cold at which it
must be maintained; typical temperatures would be in the vicinity of 110 K.
289
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290 Fundamentals of Chemical Engineering Thermodynamics
This chapter presents two examples in which we use the ideal gas law and the
van der Waals equations of state to relate P, V , and T to each other. In both examples, we use pure methane as a model for natural gas. Thermodynamic modeling
of mixtures is covered beginning in Chapter 9.
exaMPLe 7-1
sizing a PiPeLine
We are designing a new process that uses natural gas, which will be delivered via pipeline at conditions of P 5 1000 psia and T 5 1008F. In order to size process equipment,
we need to know the density of natural gas at this temperature and pressure. Use pure
methane as a model for natural gas.
Find r in lbm/ft3 for methane at P = 1000 psia and T = 1008F using:
Methane is a supercritical fluid at this
temperature and pressure; its critical point
is about P = 677 psia
and T = 21168F.
A. The ideal gas law
B. The van der Waals EOS with the parameters a = 8496 ft6 psia/lb-mol2 and b = 0.685 ft3/
lb-mol
SOLUTION A:
Step 1 Solve ideal gas law for molar volume
Temperature must always be expressed on an absolute scale when using the ideal gas law,
and T = 1008F is equivalent to T = 559.68R. Since P and T are given, the ideal gas law
can be solved immediately for V:
V5
RT
5
P
1
10.731
2
psia ? ft3
s559.68Rd
lb-mol ? 8R
1000 psia
V 5 6.005
One of the identifying
characteristics of ideal
gases is “no intermolecular interactions.”
(7.1)
ft3
lb-mol
Step 2 Convert to desired units
Density (lbm/ft3) is the reciprocal of specific volume (ft3/lbm), so we need to convert from
moles to mass. The molecular weight of methane is 16.
1
ft3
lb-mol
21
1
1
V̂ 5 6.005
r5
V̂
5
2
1 lb-mol
ft3
5 0.375
16 lbm
lbm
ft3
0.375
lbm
5 2.66
lbm
ft3
(7.2)
(7.3)
SOLUTION B:
Step 3 Apply van der Waals equation of state
The van der Waals EOS is commonly written as
P5
RT
a
2
V 2 b V2
but can be rearranged to show explicitly that it is a cubic equation in V for
The two complex
solutions to Equation 7.4 are 1.27 1
0.75i and 1.27 2 0.75i.
PV 3 2 V 2sPb 1 RT d 1 aV 2 ab 5 0
(7.4)
Here, P, T, a, and b are all known. The fact that Equation 7.4 is a cubic equation means
that it has three solutions for V. In this case, two of the solutions are imaginary. This is not
a fluke—in this chapter we will learn a method for assigning parameters to cubic equations of state in a way that produces only one real solution for temperatures above the
critical temperature. The real solution is V 5 5.293 ft3/lb-mol.
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C H A P T E R 7 Equations of State (EOS)
Converting this into density, by the same process shown in step 2, produces a result
of r 5 3.02 lbm/ft3.
The two answers are about 13% different from each other. Which one is more accurate?
Step 4 Compare to data
Examining Figure 7-1, we see the curve representing r = 3 lbm/ft3 intersects the curve representing T = 1008F almost exactly at the P = 1000 psia line, so to the limit of accuracy
of this figure, we would say the answer is r = 3 lbm/ft3. For a more accurate answer, one
5
3
0.01
0.03
0.02
7
350
300
200
250
600
2
10
100
70
50
30
20
200
2000
100
150
400
200
Pressure, psia
Based on NIST: Thermal Physics Division, Boulder, CO., as seen in Lira, J.R. Elliott and C. T., Introductory
Chemical Engineering Thermodynamics, Prentice-Hall, 1999.
500
400
450
Enthalpy, Btu/lbm
1000
800
Enthalpy, kJ/kg
1200
550
600
1400
650
1600
700
740
0.1
0.07
0.05
0.2
0.3
1
0.7
0.5
3
2
300
10
7
5
3000
1000
700
500
30
20
10,000
7000
5000
70
50
Pressure, MPa
Figure 7‑1 Thermodynamic properties of methane.
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291
292 Fundamentals of Chemical Engineering Thermodynamics
good resource is the NIST WebBook, which gives a result of r 5 2.943 lbm/ft3 (Lemmon,
2012). If we accept this value as the most accurate, then the ideal gas law is off by 9.6%,
but the van der Waals equation is only off by 2.6%.
Real natural gas is not
pure methane, but a
mixture. Methane is
the largest component
of the mixture but the
exact composition varies. Even if you have
real data for each individual component in a
mixture, you typically
don’t have data for
every possible mixture
composition. Thus, you
need models, which
rely on equations of
state.
exaMPLe 7-2
This example is oversimplified in that LNG
wouldn’t be stored in
a “rigid sealed” vessel.
LNG storage vessels
have a mechanism
for venting natural
gas that vaporizes.
No vessel is perfectly
insulated, so this
“evaporative cooling”
process sacrifices a
small amount of the
LNG in order to maintain the desired low
temperature. Evaporative cooling was previously examined in
Problem 3-22.
Example 7-1 illustrates a straightforward application for an accurate equation of state.
In chemical processes, temperature and pressure are the most commonly specified
parameters, but in order to size pipes and equipment, we often need to know the volumetric flow rate of a stream as well. This requires knowledge of the specific volume
or density, which in Example 7-1 we obtained from an equation of state. This result,
combined with a mass flow rate, would allow us to determine the volumetric flow rate.
As another application, Example 7-2 will use equations of state in a safety analysis.
A question that often arises at this point is “Wouldn’t real data be better?” and
of course the answer is yes—reliable experimental data is preferred to any theoretical estimate. However, while extensive experimental data is available for compounds like water and methane, chemical engineers cannot count on having such
data available for all chemicals they will ever work with. Furthermore, suppose your
assignment was to collect experimental data of the kind shown in Figure 7-1. In the
course of designing the experimental apparatus, you would have to size equipment
and perform a safety analysis, requiring calculations analogous to those shown in
Examples 7-1 and 7-2. Since your goal is to collect the data, it presumably doesn’t
exist yet—you would have to make estimates throughout your design process. Thus,
accurate equations of state would be needed.
When the van der Waals EOS was introduced, Example 2-6 showed that for
pressures above P ≈ 1 atm, water vapor departed significantly from ideal gas behavior, and the van der Waals equation of state provided a much better model. H2O
is an extremely polar compound that can be expected to exhibit behavior that is
not ideal. Example 7-1 illustrates that even methane, which is a spherical, non-polar
compound, departs from ideal gas behavior at high pressures and demonstrates the
improvement that results from the van der Waals EOS. In that example, the pressure
was 1000 psia or about 68 atmospheres. Example 7-2 considers conditions supercritical methane at even higher pressure.
a PressurizeD Lng stOrage cOntainer
110 lbm of liquefied natural gas (LNG) is stored in a rigid, sealed 5 ft3 vessel. Due to a
failure in the cooling/insulation system, the temperature increases to 21008F, which is
above the critical temperature; thus the natural gas will no longer be in the liquid phase.
What is the resulting pressure of the supercritical methane? If the vessel is rated as safe
for pressures of up to 500 atmospheres, will the vessel rupture? In this problem, again
model LNG as 100% methane.
Determine the answer using
A. the ideal gas law.
B. the van der Waals EOS.
The van der Waals parameters for methane are a = 8496 ft6 psia/lb-mol2 and b = 0.685
ft3/lb-mol.
SOLUTION:
Step 1 Determine number of moles of methane
Pressure of the methane is ultimately what we’re trying to determine. The temperature of the methane is known and the molar volume is readily found from the given
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C H A P T E R 7 Equations of State (EOS)
293
information. Whenever two of molar volume, temperature, and pressure of a material
are known, the third can be determined if we have an equation of state that describes
the material.
The molecular weight of methane is 16, so the number of moles of methane in the
vessel is
n5
110 lbm
M
5 6.875 lb-mol
5
lbm
MW
16
lb-mol
(7.5)
SOLUTION A:
Recall that the ideal
gas law can be written
either PV = NRT or
PV = RT, since
V = V/N.
Step 2 Apply ideal gas law
Solving the ideal gas law for pressure:
P5
NRT
5
V
1
s6.875 lb-mold 10.731
2
psia ? ft3
s359.68Rd
lb-mol ? 8R
3
5 ft
(7.6)
P 5 5306 psia
This is equivalent to approximately 361 times atmospheric pressure:
1 atm
5 361.1 atm
114.696
psia 2
P 5 s5306 psiad
The Fahrenheit to
Rankine temperature
conversion was covered in Section 1.4.8.
(7.7)
Thus the ideal gas law tells us the vessel is still safe if the methane temperature climbs
to 21008F.
SOLUTION B:
Step 3 Apply van der Waals EOS
The molar volume of the methane is
3
V5
3
5 ft
ft
5 0.7273
6.875 lb-mol
lb-mol
(7.8)
Inserting known information into the van der Waals equation gives
P5
RT
a
2
V 2 b V2
110.731 lb-mol ? 8R2s359.68Rd
(7.9)
psia ? ft3
P5
0.7273
ft3
ft3
2 0.685
lb-mol
lb-mol
8496
2
1
ft6 ? psia
lb-mol2
0.7273
FOOd FOr
thOught
7-1
Mathematically,
the extremely large
pressure can be
understood as a consequence of ( V 2 b)
being very small.
What does it mean if
V = b?
2
ft3 2
lb-mol
P 5 75,222 psia
This is over 5100 times atmospheric pressure; ten times larger than the vessel’s rated safe
pressure. We have two equations of state that are leading us to two very different conclusions. Which is right?
COMPARISON TO DATA
Figure 7-1 reveals that supercritical methane at T = 21008F and P ≈ 5600 psia has a
density of r = 22 lbm/ft3, which is exactly what we have in this example (r = 110 lbmy5 ft3).
The correct answer is thus approximately 5600 psia, or approximately 381 atmospheres
of pressure.
Normally we expect a
more complex method
to be more rigorous
and therefore more
accurate. Here that is
not the case.
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294 Fundamentals of Chemical Engineering Thermodynamics
The ideal gas estimate is off by about 5%. The van der Waals estimate is off by more
than an order of magnitude and produces the wrong conclusion—it says the vessel will
rupture while the real data says the vessel will still be safe at this temperature.
Two properties of
ideal gases are (1)
no intermolecular
interactions and (2)
particles have zero
volume.
The outcome of Example 7-2 seems to contradict what we’ve learned so far.
We’ve learned the ideal gas law is only valid at low pressures; here the actual pressure is over 300 atmospheres. We’ve learned the van der Waals equation has parameters a and b, which are intended to take into account the intermolecular interactions
between particles and the actual finite volumes of the particles. In other words, the parameters a and b are supposed to correct the exact limitations of the ideal gas model
that cause it not to work at high pressures, so we would expect the van der Waals
equation to be more accurate. In Example 7-2, however, the ideal gas law appears to
be comparatively accurate, while the van der Waals equation appears to be worthless.
The reasonable “accuracy” of the ideal gas law in this example is explained by
Figure 7-2. While the figure is specific to methane at 21008F, the qualitative behavior
shown in Figure 7-2 is typical for most compounds at most temperatures.
2
1.8
1.6
1.4
Z
1.2
1
0.8
0.6
0.4
0.2
0
0
2000
4000
6000
8000
10000
P (psia)
Figure 7‑2 Compressibility (Z) of methane vs. pressure for a
constant temperature T 5 21008F.
(Produced using data obtained from Lemmon et al. 2012.)
While the ideal gas estimate of pressure was
comparatively close,
it’s worth noting that
the estimated pressure
was 5% lower than
the real pressure. In a
safety analysis, we’d
rather overestimate
the pressure than
underestimate it.
Notice that Z = 1 (ideal gas behavior) at the low pressure limit, and Z falls below 1
as pressure increases. Z < 1 in this region because of attractive intermolecular forces
that cause the molar volume to be smaller than it would be for an ideal gas. However,
this trend of decreasing Z with increasing P cannot continue indefinitely. If molecules
are too close together, repulsive forces will exceed attractive forces. At some point,
the molecules are compressed so close together that increasing P has little further
effect on V. Since Z = PVyRT, if P is increased while V remains nearly constant, Z
must also increase. At P ≈ 5400 psia, Z = 1, not because the attractive and repulsive
phenomena are negligible (as they would be for an ideal gas), but because this happens to be the one pressure at which they cancel each other out. The example was
deliberately designed to fall near this pressure. Problem 7-8 looks at a variant on this
example in which the ideal gas law prediction is far worse than it was in Example 7-2.
The “failure” of the van der Waals equation can be explained simply by saying
the van der Waals equation isn’t flexible enough to be used at all conditions. The
numerical values of a and b that we used in Example 7-1 were determined by fitting
the equation to critical point data for methane. This method is illustrated in detail
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C H A P T E R 7 Equations of State (EOS)
295
in Example 7-4. If we had used other data (e.g., the data in Figure 7-2) as our starting point, we would have gotten different values for a and b. Alternative values of a
and b might have worked better for this particular example, but there simply is no
single set of parameters a and b that accurately model methane at all temperatures
and pressures. Methane is a fairly simple compound: It is not polar, symmetrical, and
contains only two different elements (C and H) and one kind of bond (the C—H
single bond.) If the van der Waals equation doesn’t describe methane satisfactorily,
one can’t expect it to be useful for larger, more complex compounds.
The van der Waals equation was first proposed in 1873, and at that time, it represented a breakthrough in the understanding of chemical compounds and the ability
to build mathematical models. However, it has since been replaced by more accurate
(and more complex) equations. Modern equations of state, and their use, are presented in this chapter.
7.2 cubic equations of state
The van der Waals equation is a cubic EOS—it is first degree in T and P but third
degree in V. Consequently, for a given T and P, there are three solutions for V
(although in some circumstances such as Example 7-1 two solutions are imaginary).
Although the van der Waals equation is no longer in common use, we will see in this
chapter that there are equations of state in common use that are more complex variants of the van der Waals equation. This section outlines how values of adjustable parameters (such as a and b in the van der Waals equation) are determined, how cubic
equations of state can be solved, and how they can be used in combination with the
mathematical techniques described in the previous section. We begin by addressing
the question: Why use cubic (third degree) equations, specifically?
7.2.1 the rationale for cubic equations of state
Consider Figure 7-3, which shows a P-V diagram for H2O. Imagine that we have a
piston-cylinder device containing 1 mole of water vapor initially at P = 1 bar and
T = 3008C. We gradually lower the piston, compressing the vapor into a smaller
volume. At the same time, we maintain a constant temperature of T = 3008C. The
T = 3008C isotherm in Figure 7-3 helps illustrate what is observed.
At T = 3008C and P = 1 bar, H2O is a vapor with a molar volume of V =
47.54 L/mol.
As we decrease the volume at constant temperature, the pressure of the vapor
increases, as illustrated by curve AB on Figure 7-3.
This continues until we reach P = 85.88 bar. At P = 85.88 bar and T = 3008C,
we have a saturated vapor with molar volume V = 0.391 L/mol, as indicated as
point B on Figure 7-3.
If we continue to compress the saturated vapor into a smaller volume, the pressure will not increase. Instead, vapor will begin to condense into saturated liquid,
which has a molar volume of 0.0253 L/mol, as indicated by point C. Point C is
also shown on Figure 7-4, which contains the same data shown in Figure 7-3 but
focuses on the small molar volumes associated with liquids.
As the volume of the cylinder decreases, the proportion of liquid will increase. For example, when the molar volume of the two-phase mixture is V =
0.208 L/mol, it will be 50% vapor and 50% liquid: 0.208 L/mol = 0.5(0.391 L/
mol) + 0.5(0.0253 L/mol).
Phase changes and the
theory that explains
why they occur at
specific temperatures
and pressures are discussed in Chapter 8.
In this calculation the
quality (q) of the VLE
system is 0.5.
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296 Fundamentals of Chemical Engineering Thermodynamics
D
200
180
T 5 350 (liq)
160
140
P (bar)
120
T 5 300 (liq)
100
C
B
80
T 5 250 (liq)
60
T 5 400
T 5 350 (vap)
40
A
20
T 5 300 (vap)
0
0
0.5
1
V (L/mol)
1.5
2
T 5 250 (vap)
Figure 7‑3 The P-V diagram for liquid and vapor H2O at 250, 300, 350,
and 4008Celsius.
500
450
400
FOOd FOr
thOught
7-2
This entire section
on equations of state
focuses on liquids and
vapors. Why are solids
not considered?
P (bar)
350
T 5 2508C (liq)
T 5 3008C (liq)
T 5 3508C (liq)
Sat. liquid
300
250
D
200
150
100
C
50
0
0
0.01
0.02
0.03
0.04
0.05
0.06
V (L/mol)
Figure 7‑4 The P-V diagram for H2O at various temperatures,
focusing on the liquid phase.
In Figure 7-3 the three
curves representing
liquid properties at
250, 300, and 3508C
look like vertical lines
that are indistinguishable from each other.
Figure 7-4 focuses on
small values of V to
show the liquid phase
behavior more clearly.
When the molar volume reaches V = 0.0253 L/mol, the last bubble of vapor will
condense. We will then have a pure, saturated liquid at T = 3008C. All of the
H2O is now described by point C on Figures 7-3 and 7-4.
If we continue lowering the piston, the pressure will increase extremely sharply. For
example, the pressure at point D (P = 200 bar and V = 0.0245 L/mol) is more than
double that of saturated liquid, but the molar volume of the liquid has changed by
only ≈3%. Thus, on Figures 7-3 and 7-4, curve CD is very nearly a vertical line.
If you select a different temperature, say T = 1008C or T = 3508C, and repeat this
experiment, you will observe essentially the same sequence of events, although the
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C H A P T E R 7 Equations of State (EOS)
tabLe 7-1 Saturation conditions for H2O at selected temperatures.
temp.
(8c)
vapor
Pressure (bar)
saturated vapor Molar
volume V v (L/mol)
saturated Liquid Molar
volume V L (L/mol)
100
1.013
30.12
0.0188
1600
200
15.55
2.29
0.0208
110
300
85.88
0.391
0.0253
15.5
350
165.3
0.159
0.0313
5.08
370
210.4
0.0901
0.0399
2.26
373.95
220.6
0.0560
0.0560
1
V v/V L
numerical values of molar volume, saturation pressure, etc. might be very different,
as illustrated in Table 7-1. However, if you choose a high enough temperature, say
T = 4008C, the phase transition does not occur; the pressure simply increases continuously as molar volume decreases. This is because the critical temperature of H2O is
about T = 3748C. At temperatures higher than this, water vapor will not condense—
regardless of pressure—so no two-phase equilibrium mixture is possible.
Figure 7-3, Figure 7-4, and Table 7-1 represent the actual, observed behavior of
H2O. While the specific numbers vary greatly from one compound to another, qualitatively, these figures are representative of P-V-T behavior for most any chemical
compound. Thus, they represent the kind of data we are trying to model when we
propose an equation of state. Let us now consider what sort of equation would work.
We can easily imagine fitting an equation to curve AB in Figure 7-3, and thus
modeling P vs. V for water vapor at T = 3008C. We can imagine building temperature
dependence into the equation, so that it models the vapor phase at all temperatures—
rather than just T = 3008C specifically. We can also imagine fitting an equation to curve
CD; thus, modeling P-V for water in the liquid phase. However, practically speaking, it
would be useful to model both the liquid and the vapor with a single equation.
Figure 7-5 shows an attempt to fit a curve to the data from the T = 3008C isotherm. For a P-V function to include both curves AB (vapor phase) and CD (liquid
200
D
Pressure (bar)
150
F
100
C
50
B
A
0
Function
Liquid data
0
0.5
1
1.5
250
2
Vapor data
2100
2150
297
Table 7-1 illustrates
that saturated liquid
and saturated vapor
properties converge
and become identical
at the critical point.
At temperatures far
below the critical
point, V L is insignificant compared to V V.
This observation will
form a basis for the
Clausius-Clapeyron
equation in Chapter 8.
The critical point was
first described in
Section 2.2.1.
Chapter 6 showed
that we can use equations of state to model
changes in enthalpy,
entropy, internal
energy, etc., but the
theory in Chapter 6
is based on partial
differential calculus.
You may recall from
calculus that you can’t
take a derivative of
a function unless it is
continuous.
E
2200
Molar Volume (L/mol)
Figure 7‑5 A continuous function that models the P vs. V data for
liquid water and water vapor at T = 3008C (curves AB and CD in
Figure 7-3).
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298 Fundamentals of Chemical Engineering Thermodynamics
fOOD fOr
thOught
7-3
A portion of the
function representing
pressure in Figure 7-5
is negative. Isn’t this
physically unrealistic?
phase) and be continuous, the region in between must have a maximum and a minimum, as illustrated in Figure 7-5. Consequently, if a single, continuous equation of
state is to be used to describe both the liquid and the vapor phases, it must be (at
least) a third-degree equation in V. While these data are for water at T = 3008C, the
phenomenon is universal: In order to model P-V-T for both the liquid and vapor
phases with a single equation for any compound, it must be an equation of at least
degree 3. Cubic equations have proven to work well and are in common use, as discussed throughout the remainder of Section 7-2.
7.2.2 Modern cubic equations of state
The van der Waals equation, introduced in Chapter 2, is the oldest cubic EOS. It can
be written as in Equation 2.53:
P5
The compressibility
factor (Z ) was first defined in Section 2.3.4,
where Z = PV/RT
RT
a
2 2
V2b V
or in terms of the compressibility factor Z (Equation 2.55):
Z5
V
V2b
2
a
RT V
Peng and Robinson (Robinson, 1976) describe the two terms on the right-hand side
of Equation 2.53 as the “repulsion pressure” and the “attraction pressure,” so the van
der Waals equation can be viewed as
P 5 P rep 1 P att
(7.10)
with
P rep 5
The Peng-Robinson
equation was used
to fit the data in
Figure 7-5.
FOOd FOr
thOught
7-4
Would it make sense
to propose an equation of state that was
third degree in P or T,
rather than V?
RT
V2b
and P att 5 2
a
V2
(7.11)
The “repulsion pressure” is proportional to T, because as temperature increases,
molecules move more quickly and collisions become more common. Because two
particles cannot occupy the same space, a fluid has a minimum molar volume represented by b. As the actual molar volume V gets very close to the minimum possible
volume b, the repulsion pressure goes toward infinity.
The “attraction pressure” is intended to model the effect of intermolecular attractive forces, which become less important when the molecules are father apart;
hence, the term is proportional to 1yV 2.
The motivational example illustrated two things: (1) there’s a practical need for
equations of state that allow us to accurately relate P, V, and T to each other and
(2) the van der Waals EOS is not acceptably accurate, at least not for supercritical
methane. The most useful EOS is a single equation that describes the liquid, vapor,
and supercritical states at all realistic temperatures and pressures.
While the van der Waals equation is no longer in common use, it has proved to
be a useful foundation for equations of state:
The use of an equation that is cubic in V has stood the test of time for reasons
that were explored in Section 7.2.1.
The idea of an “attraction” term and a “repulsion” term with two adjustable
parameters has also stood the test of time. In subsequent equations, these parameters have typically been called “a” and “b” for consistency with the van der
Waals equation.
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C H A P T E R 7 Equations of State (EOS)
299
The van der Waals “repulsion pressure” term RTy (V 2 b) reappears explicitly
and unmodified in several subsequent equations of state.
The innovations that have been added since the van der Waals EOS was published
in the 1870s are
The parameter a, rather than being a constant, is a function of temperature.
The parameter b, which represents the molar volume of a substance at maximum compression, appears in the “attraction” term as well as the “repulsion”
term.
Modern cubic equations of state vary from each other primarily in the specific mathematical form of the “attraction” term. Two of the most prominent examples are the
Soave equation (Soave, 1972) and the Peng-Robinson equation (Robinson, 1976),
and these will be used as the primary cubic equations of state throughout the remainder of this book.
The Soave equation is
P5
RT
a
2
V 2 b VsV 1 bd
(7.12)
which, in terms of the compressibility factor, is
Z5
V
V2b
2
1RTa 21V 11 b2
(7.13)
The “repulsion pressure” term is identical
in the van der Waals,
Peng-Robinson, and
Soave equations.
The “attraction pressure” terms are more
complex in the PengRobinson and Soave
equations than in the
van der Waals equation, and these newer
equations of state typically produce much
better answers.
The Peng-Robinson equation is
P5
RT
a
2
V 2 b VsV 1 bd 1 bsV 2 bd
(7.14)
which, when written in terms of the compressibility factor, is:
Z5
1
1
23
V2b
RT VsV 1 bd 1 bsV 2 bd 4
V
2
aV
(7.15)
The development of these equations will not be discussed, as it was primarily
empirical—many equations have been tried and these have proved to work particularly well for a broad range of compounds. In both the Peng-Robinson and Soave
equations, b is a constant, but a is a function of temperature. In Section 7.2.3 and
Section 7.2.4, we assume the parameters a and b are known, examine how to solve
the equations, and interpret the solutions. In Section 7.2.5, we describe the temperature dependence of a and show how values for a and b can be determined when
they are not known.
7.2.3 solving cubic equations of state
An equation of state has been defined throughout this book as a mathematical relationship among P, V, and T, such that if any two are known, the third can be found.
The van der Waals, Peng-Robinson, and Soave equations are conventionally written
in the form P = f (T, V ) or Z = f (T, V ) and are readily solved if P is the unknown.
Solving for T looks like a simple algebraic manipulation at first glance, since there
is only one T explicitly shown in Equations 7.13 and 7.15, but keep in mind that in
the Peng-Robinson and Soave equations, a is not a constant, it is a function of T.
The Soave equation
is also often referred
to as the SoaveRedlich-Kwong, or
Redlich-Kwong-Soave
equation, because of
its similarity to an
equation Redlich and
Kwong published in
1949.
Peng and Robinson
(Robinson, 1976) used
the word “semiempirical” to describe the
van der Waals EOS
and all of the cubic
equations (including
their own) stemming
from it.
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300 Fundamentals of Chemical Engineering Thermodynamics
This section, however, will focus on solving for V, which is the variable for which the
equations are cubic.
Cubic equations do not lend themselves to simple algebraic solutions, but they
can be solved analytically, as described in Appendix B. However, modern computing
means that engineers rarely need to implement this analytical solution approach “by
hand.” Indeed, solving cubic equations in practice is much simpler than it was even
a decade or two ago, as many modern calculators are capable of solving cubic equations with no input required from the user beyond the equation itself. Mathematical
software packages can also be used (Cutlip, 2007).
Example 7-3 illustrates a graphical solution approach that can be readily
implemented using any conventional spreadsheet application, such as Microsoft
Excel.
exaMPLe 7-3
The value of a in this
example is specific to
the temperature T =
1008C. Section 7.2.4
discusses how the
numerical values of a
and b are determined
for the van der Waals,
Peng-Robinson, and
Soave equations of
state.
DeterMining MOLar vOLuMe Of Pentane using a cubic equatiOn
Of state
Find the molar volume of n-pentane at T = 1008C and P = 1 bar. The Peng-Robinson
parameters for n-pentane at this temperature are: a = 2.417 × 107 bar ? cm6/mol2 and
b = 90.18 cm3/mol.
SOLUTION:
Step 1 Solve Peng-Robinson equation for one value of V
The Peng-Robinson equation can be expressed as
P5
a
RT
2
V 2 b VsV 1 bd 1 bsV 2 bd
Temperature must be expressed on an absolute scale, so T = (100 + 273.15) K = 373.15 K.
While there is no algebraic way to rearrange the equation so it is explicit in V, we can
easily find the P that corresponds to any V. For example, if V = 100 cm3/mol,
? cm
s373.15 Kd
183.14 bar
mol ? K 2
P5
cm
cm
1100 mol 2 90.18 mol2
3
3
(7.16)
3
2.417 3 107
2
1
100
cm3
mol
21
100
bar ? cm6
mol2
2 1
cm3
cm3
cm3
1 90.18
1 90.18
mol
mol
mol
21
100
cm3
cm3
2 90.18
mol
mol
2
P 5 1944.8 bar
This is not a useful result by itself; we are trying to find the solution for P = 1 bar. However, if we enter Equation 7.16 into a spreadsheet, solving for several hundred different
values of V takes little more effort than solving for one.
Step 2 Solve for P over a range of values of V
We solve for P at a wide range of values for V and identify the values that correspond to
P = 1 bar. Table 7-2 shows a small subset of the data, and Figure 7-6 shows plots.
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C H A P T E R 7 Equations of State (EOS)
tabLe 7-2 Solutions of Equation 7.16 for different values of V.
V (cm3/mol)
P (bar)
133
3.996
134
23.055
135
29.574
136
215.603
465
20.01328
470
0.448748
475
0.895148
480
1.326498
30000
1.010546
30200
1.004008
30400
0.997554
30600
0.991182
100
50
0
0
500
1000
250
2100
Molar volume (cm3/mol)
Pressure (bar)
Pressure (bar)
100
50
0
0
5000
10000
250
2100
Molar volume (cm3/mol)
Figure 7‑6 P-V plots for the T = 1008C isotherm, as calculated for
n-pentane using the Peng-Robinson equation.
Step 3 Identify the solutions that correspond to P = 1 bar
While step 2 produced a complete isotherm for T = 1008C, we were asked to find the
molar volume at P = 1 bar. There are three molar volumes that correspond to a pressure of 1 bar; Table 7-2 reveals that one of them falls between 133 to 134 cm3/mol, one
between 475 to 480 cm3/mol, and one between 30,200 to 30,400 cm3/mol. The exact values of V can be found using the “Solver” function of Microsoft Excel, or even by trialand-error until one finds values of V that yield P = 1 bar to as many significant digits
as desired. In this case, the solutions are V = 133.42 cm3/mol, V = 476.20 cm3/mol, and
V = 30,324 cm3/mol.
A graphical approach can be applied to any equation of state and any temperature. If
the temperature of interest is below the critical point, and the isotherm produced does
not qualitatively resemble Figure 7-6, most likely you either made an error in entering
the equation, or your range of V values is simply not sufficient to reveal all three roots.
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301
302 Fundamentals of Chemical Engineering Thermodynamics
While the solution illustrated in Example 7-3 is a “brute force” approach that may
seem crude compared to an analytic solution or an efficient numerical solution algorithm, it can be implemented rapidly with modern spreadsheets, and it has instructional
value. Rather than simply giving three answers as a calculator would do, the graphical approach allows one to examine the full behavior of the equation. Thus, using the graphical
approach (at least some of the time) can help you develop an intuitive feel for equations
of state. A graphical approach can also help resolve confusion. For example, if you are expecting three solutions and your calculator returns only one, there are many possibilities:
Was your expectation wrong? Did you enter the equation wrong? Are the three roots
identical? Did the calculator fail to detect two roots? Making a full graph of the equation
can help you answer such questions.
7.2.4 interpreting solutions to cubic equations of state
Figure 7-7 illustrates solutions to a cubic EOS in the form of P vs. V, for various values of T. For temperatures above the critical point, P simply decreases as V increases.
For lower values of T, the isotherm has a maximum and a minimum. Consequently,
over a large range of P values, there are three real values for V that each produce
the same P. Section 7.2.3 discussed how to find these three real solutions. The natural
next question is “If we have three solutions, which one is right?”
Examining Figure 7-5 in Section 7.2.1 reveals that a significant portion of the function doesn’t correspond to any real data. Part of this function has negative values of
pressure, which doesn’t make physical sense. This is a commonly, but not always, observed feature of cubic equations of state. Another unrealistic feature of part of the
function appears on curve EF, where molar volume is actually increasing as pressure
increases. This doesn’t make physical sense, but it is a feature of all cubic equations of
state; the portion of the curve that “joins” the liquid and vapor data includes a region
Pressure
T . Tr
T 5 Tr
T , Tr
Molar volume
Figure 7‑7 Isotherms generated using a cubic equation of state.
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C H A P T E R 7 Equations of State (EOS)
303
in which V increases with increasing P. The requirement that molar volume must decrease as pressure increases isothermally can be stated mathematically as
1 −P 2 , 0
−V
(7.17)
T
Equation 7.17 is often called the stability criterion; if the solution to an equation of
state at a particular point doesn’t meet this criterion, it doesn’t describe a stable phase.
Revisiting Example 7-3, we determined that for n-pentane at P = 1 bar and
T = 1008C, the Peng-Robinson equation produces three solutions for molar volume:
V = 133.42 cm3/mol, V = 476.20 cm3/mol, and V = 30,324 cm3/mol. When we look at
the form of the data we’re actually modeling (e.g., Figure 7-3, Figure 7-5), these three
solutions seem to lend themselves to a simple physical interpretation. The smallest V
(133.42 cm3/mol) corresponds to a liquid V, the largest (30,324 cm3/mol) corresponds
to a vapor V, and the intermediate one (476.20 cm3/mol) has no physical significance.
As illustrated in Section 7.2.1, we are using a continuous function to model data that
is not continuous, and the point P = 1 bar, V = 476.20 cm3/mol appears on the portion of the curve that violates the stability criterion.
However, Figure 7-5 also reveals that there are two sections of the curve (CE
and FB ) that meet the stability criterion but still don’t represent the data. Curve CE
represents the liquid phase, but at pressures below the vapor pressure at which liquid
does not exist—at least not at equilibrium. Similarly, curve FB represents the vapor
phase—but at pressures above the vapor pressure. The word metastable is used to
describe the conditions represented by curves CE and FB.
According to Brown (2011), the experimental vapor pressure of n-pentane is
about P sat = 5.93 bar at T = 1008C. Thus, at P = 1 bar and T = 1008C, n-pentane
exists in the vapor phase, and its molar volume is V = 30,324 cm3/mol. The value
V = 133.42 cm3/mol is a metastable solution—for pure pentane at this temperature
and pressure it isn’t physically realistic either.
Thus, while “the smallest V is the liquid molar volume” sounds like a simple and
logical interpretation, we need to be a little more specific and say that in the threesolution region:
If there is a stable liquid phase, its molar volume is represented by the smallest
value of V.
If there is a stable vapor phase, its molar volume is represented by the largest
value of V.
The vapor pressure is the only pressure at which the liquid and vapor values of
V both represent stable phases.
The middle value of V never represents a stable phase.
In this analysis, we determined which of the three roots for V was metastable and
which was “right” by comparing the actual pressure to the vapor pressure. What if
the vapor pressure had been unknown? Chapter 8 addresses this question, discussing methods of estimating vapor pressure. Indeed, it is possible to compute vapor
pressures directly from an equation of state, as illustrated in Example 8-5.
To close, we note that real compounds can be observed in metastable conditions
in certain circumstances. For example, if steam at P = 1 atm and T = 1008C is either
cooled by a fraction of a degree or compressed to a fractionally higher pressure, the
steam should condense—liquid is the most stable state at the new T and P. However,
condensation requires nucleation sites—locations where the liquid phase begins to
form. Steam at 99.58C and 1 atm is thus an example of a “metastable” state. It would
Pure n-pentane is a
vapor at T = 1008C
and P = 1 bar, but
n-pentane can exist in
the liquid phase
at T = 1008C and
P = 1 bar if it is a
component of a
mixture. Chapter 9
discusses this distinction further.
You may recall from
introductory chemistry that chemical
reactions have an
“activation energy”
required to form the
transition state. You
can think of the nucleation site in a phase
change as analogous
to the transition state
in a chemical reaction.
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304 Fundamentals of Chemical Engineering Thermodynamics
not exist at equilibrium, but it could exist temporarily until the nucleation process
begins. In this introductory book, however, we are only concerned with states that
are stable at equilibrium and will not substantially address “metastable” states or
how this nucleation process occurs.
7.2.5 Fitting Parameters to cubic equations of state
This section has discussed the van der Waals, Soave, and Peng-Robinson equations
and their solutions. Up to this point, the parameters a and b have simply been given.
We now examine the question of how specific values are assigned to these parameters.
In Chapter 2, we introduced the physical uniqueness of the critical point. Below
its critical temperature, a compound can exist in the liquid, solid, or vapor state;
above its critical temperature, the compound does not condense into the liquid
phase, regardless of pressure. Figure 7-7 illustrates the solutions to a cubic equation
of state at various temperatures.
The curves in Figure 7-7 are isotherms; they represent P as a function of V with
T held constant. Thus, the derivative of one of these curves is (−Py−V)T .
Figure 7-7 shows the uniqueness of the critical point in the context of a mathematical model.
For temperatures above the critical point, (−Py−V )T is always negative; pressure
decreases continuously as molar volume increases.
For temperatures below the critical point, each isotherm has a maximum and a
minimum—two points at which (−Py−V )T = 0. While these points occur on the
portion of the curve that does not represent observed data (see Section 7.2.4),
they are a recognizable aspect of our mathematical model.
Recall from calculus
that at a saddle point,
the first and second
derivatives (dy/dx and
d2y/dx2) are both zero.
exaMPLe 7-4
Mathematically, the transition between these two behaviors takes the form of a single
temperature at which there is a saddle point. Notice in Figure 7-7 that as the temperature approaches the critical temperature, the maximum and minimum points on the
isotherms get closer together; you can think of the saddle point as the point where
the maximum and minimum “meet.” Thus, a cubic EOS has only one saddle point,
and to be physically realistic, it must occur at the critical point. This observation has
tremendous significance in model-building. Mathematically, for a function y = f (x),
at a saddle point, the first and second derivatives dyydx and d 2yydx2 both equal 0.
Example 7-4 considers how this observation can be applied to the van der Waals EOS.
fit ting ParaMeters a anD b fOr the van Der WaaLs equatiOn
tO criticaL POint Data
The critical point of water is Tc = 647.3 K and Pc = 221.2 bar. Determine the van der
Waals parameters a and b for water, and also find the compressibility of water at its critical point (Zc ). Evaluate the results using data from the steam tables.
SOLUTION: As discussed previously, for a cubic equation of state in the form P =
f (T, V ), the isotherm representing T = Tc will have a saddle point located at the critical
point. This means that at the critical point, (−Py−V )T and (−2Py−V 2)T will both equal 0. To
apply these facts, we must first evaluate the derivatives.
Step 1 Differentiate the van der Waals equation with respect to V
Differentiating the van der Waals equation with respect to V while holding T constant
produces
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C H A P T E R 7 Equations of State (EOS)
P5
RT
a
2 2
V2b V
1−V 2 5 sV 2 bd 1 V
2RT
−P
2a
2
T
3
(7.18)
and
1−V 2 5 sV 2 bd 2 V
−2P
2RT
2
6a
4
3
T
(7.19)
At the critical point (T = Tc ), as noted, these derivatives are both zero:
1−V 2 5 sV 2 bd 1 V 5 0
(7.20)
1 2
(7.21)
2RTc
−P
2a
3
c
2
T
c
and
2RTc
−2P
6a
50
5
2
−V 2 T sV c 2 bd3 V 4c
Step 2 Consider degrees of freedom
Since Tc and Pc are both known (as is the constant R), Equations 7.20 and 7.21 contain
three unknowns: the parameters a and b and the critical volume V c. We can obtain a
third equation by using the EOS itself. If we expect the van der Waals equation to be a
reasonable model for liquids, vapors, gases and supercritical fluids at all temperatures
and pressures, then we certainly expect it to be valid at the critical point. Thus, there are
three equations and three unknowns: the values of a and b can be determined using no
information beyond the critical pressure and temperature.
Step 3 Express V in terms of b
Equations 7.20 and 7.21 can be rearranged into
RTc
sV c 2 bd2
5
2a
V 3c
(7.22)
5
6a
V 4c
(7.23)
and
2RTc
sV c 2 bd
3
These equations are greatly simplified when we divide Equation 7.23 by Equation 7.22 for
2
3
5
sV c 2 bd V c
(7.24)
V c 5 3b
(7.25)
which simplifies to
Step 4 Express a as a function of b
Plugging the expression for V c given in Equation 7.25 into Equation 7.22 gives
RTc
s3b 2 bd2
5
2a
s3bd3
(7.26)
This can be solved for a, as
a5
27
RTc b
8
(7.27)
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305
306 Fundamentals of Chemical Engineering Thermodynamics
Step 5 Solve the equation of state for b
Substituting the expressions for a and V (Equations 7.25 and 7.27) into the van der Waals
EOS produces:
RTc
Pc 5
Vc 2 b
2
a
V 2c
27
RTb
8
2
Pc 5
3b 2 b
s3bd2
RTc
RTc
Pc 5
2
2b
b5
(7.28)
3RTc
8b
RTc
(7.29)
8Pc
Step 6 Solve for a and Zc
Back-substituting the expression for b into Equations 7.27 and 7.25 gives
2
a5
2
27 R T c
64 Pc
(7.30)
3 RTc
8 Pc
(7.31)
and
Vc 5
We were asked to determine the compressibility factor at the critical point. Recall that by
definition, the compressibility factor is Z = PVyRT. The value of Zc is obtained directly
from Equation 7.31 as
Zc 5
Pc V c
RTc
5
1
Pc
2
3 RTc
5 0.375
RTc 8 Pc
(7.32)
Step 7 Plug in known information
The outcome of step 6 is that Zc, which is the compressibility at the critical point, is not
dependent upon the critical temperature and pressure; it is simply equal to 0.375. Inserting the known values of critical temperature and pressure into Equations 7.29 and 7.30 to
find a and b for water gives
2
a5
a5
27
64
1
83.14
2
bar ? cm3 2
s647.3 Kd2
mol ? K
221.2 bar
b5
b5
1
2
27 R T c
64 Pc
83.14
5 5.52 3 10 6
bar ? cm6
mol2
(7.33)
RTc
8 Pc
2
bar ? cm3
s647.3 Kd
mol ? K
8s221.2 bard
5 30.4
cm3
mol
(7.34)
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C H A P T E R 7 Equations of State (EOS)
307
Step 8 Compare to published information
The calculated Zc in this example is not very good; we can determine from the steam
tables that Zc for water is only 0.233. This discrepancy is discussed further after the
example.
We can evaluate the van der Waals constants, a and b, by comparing predictions
computed using these values to data. For example, Appendix A shows that at T = 1008C,
saturated liquid water has a specific volume V̂ L = 0.001043 m3/kg, and saturated vapor
has V̂ V = 1.6718 m3/kg. Converting to a molar basis gives
1
V L 5 0.001043
1
V V 5 1.6718
m3
kg
21
21
21
2
100 cm 3
cm3
5 18.79
m
mol
(7.35)
2 11000 g2 1 mol 2 1 m 2 5 30,117 mol
(7.36)
m3
kg
1 kg
1000 g
1 kg
18.015 g
mol
18.015 g
100 cm 3
cm3
Using the van der Waals equation to compute pressure corresponding to these values of
V gives
P5
a
RT
2
V 2 b V2
183.14 mol ? K 2s373.15 Kd 5.52 3 10 barmol? cm
2
5 218,307 bar (7.37)
P 5
cm
cm
18.79 2 30.4
18.79
1 mol2
mol
bar ? cm3
6
6
2
L
3
3
2
? cm
bar ? cm
s373.15 Kd 5.52 3 10
183.14 bar
mol ? K 2
mol
2
5 1.025 bar
P 5
cm
cm
30,117 2 30.4
130,117 mol2
mol
3
6
6
2
V
3
3
2
(7.38)
The pressure P L that was computed from V L is a negative number; it’s physically unrealistic. The source of the problem is our computed value of b. This parameter is intended
to represent the molar volume of the molecule at “maximum compression,” which is the
lowest value V can possibly take. However, this value of b is larger than the experimental
values of V L that are observed at most any conditions.
Meanwhile, the pressure PV that we computed from V V (1.025 bar) is quite close to
the familiar experimental value (P sat = 1.013 bar at T = 1008C), but this is not really because the values of a and b are “good”—it’s more that they have little effect for a vapor
at this low pressure. The ideal gas law for this T and V produces P = 1.03 bar; our answer
is only fractionally different than it would have been if a = b = 0.
What we must remember is that real fluids do not “obey” our equations of state;
rather, we build mathematical models that attempt to describe real behavior as closely
as possible. Our calculations in steps 1 through 7 are correct if you start from the critical
point, but the answer is only useful to the extent that our original assumption—that the
van der Waals equation describes P-V-T relationships for water—is correct. There simply
is no single set of values for a and b that accurately model H2O at all conditions.
However, with the more flexible Soave and Peng-Robinson equations, it is possible
to more accurately model the P-V-T behavior of many compounds at a wide range of
conditions. Values of the parameters a and b can be found through examination of the
critical point, as was done in this example.
ALL of the cubic
equations of state
examined in this section simplify to the
ideal gas law when
a = b = 0.
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308 Fundamentals of Chemical Engineering Thermodynamics
Notice the similarity between values of
Zc, especially among
groups of similar compounds like ethane,
propane, and butane,
or ethanol, propanol,
and isopropanol. This
is a basis for the principle of corresponding
state described in
Section 7-3.
Ethylene, propylene, and butadiene
are among the top
chemicals in annual
worldwide sales by
mass, because they
are commonly used
as intermediates for
synthesizing larger
compounds. The
double bonds provide
an active site for
either polymerization
or adding new
functional groups.
tabLe 7-3 Critical properties of various compounds.
compound
Tc (K)
Pc (bar)
Zc
Water
647.3
221.2
0.233
Oxygen
154.6
50.43
0.288
Carbon dioxide
304.2
73.82
0.274
Freon-22
369.8
49.70
0.268
Ethylene
282.4
50.32
0.277
Propylene
364.8
46.13
0.275
1,3-Butadiene
425.4
43.30
0.270
Ethanol
516.4
63.84
0.248
Propanol
536.7
51.70
0.253
Isopropanol
508.3
47.64
0.248
Ethane
305.4
48.80
0.284
Propane
369.8
42.49
0.281
n-Butane
425.2
37.97
0.274
Isobutane
408.1
36.48
0.282
While Example 7-4 examined H2O, no information specific to H2O was used in steps
2–6. Thus, Equations 7.27, 7.29, and 7.32 are, mathematically speaking, valid for any
compound. Equation 7.32 essentially says that, IF the vapor and liquid states of all
real compounds were accurately described by the van der Waals EOS, then all real
compounds would have a compressibility of 0.375 at their critical point. Table 7-3
illustrates that this outcome leaves much room for improvement, as Zc is typically
smaller than 0.375 and is clearly not identical for all compounds. This data is another
illustration of why the van der Waals equation is no longer in common use. The data
does, however, illustrate van der Waals’ substantial improvement upon the ideal gas
law, which says that Z = 1 at the critical point and at all other conditions.
The strategy of fitting parameters for an EOS by applying mathematical principles to the critical point can also be applied to the more modern cubic equations.
Full derivations will not be shown, but the approach is analogous to that shown in
Example 7-4 for the van der Waals equation.
In the Soave equation, the value of b is equal to
b 5 0.08664R
Note that the forms
of Equations 7.39 and
7.48 are identical to
that of 7.29, although
the constants relating b to Pc and Tc are
different.
Based on data from Lira, J.R. Elliott and C. T., Introductory Chemical Engineering Thermodynamics, Prentice-Hall, 1999.
The data in Table 7-3
was not available
when van der Waals
originally proposed
his equation of state
in 1873. In fact, the
concept of a “critical
point” was itself very
new at that time.
Tc
Pc
(7.39)
In the Soave equation, the parameter a is a function of temperature. However, the
methodology outlined in Example 7-4 can be used to determine the value of a at the
critical point (ac):
ac 5 0.42747R2
T 2c
Pc
(7.40)
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C H A P T E R 7 Equations of State (EOS)
309
Soave used the variable to quantify the temperature dependence of the parameter
a; is the ratio of a at the temperature of interest to a at the critical temperature:
5
a
ac
(7.41)
Soave found that could be well expressed as a function of the reduced temperature
(Soave, 1972), as
0.5 5 1 1 m ( 1 2 T 0.5
)
r
(7.42)
where the reduced temperature (Tr ) is defined as the ratio of the actual temperature
to the critical temperature:
Tr 5
T
Tc
(7.43)
This is the first time we have encountered the reduced temperature. The analogous
reduced pressure (Pr ) is defined in Equation 7.44. We will find that Tr and Pr are useful in model building, particularly in Section 7.3 when we develop the principle of
corresponding states.
Pr 5
P
Pc
Recall that in algebra,
a straight line is conventionally expressed
in the form y = mx + b.
Soave used the symbol
m in Equation 7.42,
presumably because
he noted that the relationship between 0.5
and Tr0.5 was in practice almost linear.
(7.44)
Returning to Soave’s EOS, Equation 7.42 is the expression for as Soave originally
published it. In practice, one usually calculates m and Tr first and then finds from
these, so we prefer a form that gives explicitly.
5 [ 1 1 m ( 1 2 T 0.5
) ]2
r
(7.45)
Finally, Soave related the value of m to the acentric factor as
m 5 0.480 1 1.574 2 0.1762
(7.46)
The acentric factor is
1PP 2 *
sat
5 21 2 log 10
c
(7.47)
Tr 50.7
Thus, the acentric factor is derived from a single data point—if you know the vapor
pressure of a compound at the reduced temperature of Tr = 0.7, you can determine
the acentric factor using Equation 7.47. The rationale for this definition, and for its
use in an EOS, is provided in Section 7.2.6. For now, the acentric factor is a state property that has been tabulated for many compounds. It is often published alongside
critical properties (Tc and Pc) since these three properties are sufficient to allow one
to model a compound in the liquid, vapor, and supercritical states using a cubic EOS.
Peng and Robinson (Robinson, 1976) developed their equation of state in a very
similar way to Soave, although because the “attraction” terms in the two equations
are different, the exact equations relating ac and b to the critical temperature and
pressure are slightly different. In the Peng-Robinson equation,
b 5 0.07780R
Tc
Pc
(7.48)
and
ac 5 0.45724R2
T 2c
Pc
(7.49)
Kenneth Pitzer was
the primary author
of a pair of articles
(Pitzer, 1955 and
Pitzer et al., 1955)
that introduced the
acentric factor, which
is sometimes called
the “Pitzer acentric
factor.”
For Soave and PengRobinson, there are
several equations and
several steps involved
in finding the value
of the parameter a at
a particular temperature, but there are only
three pieces of data
you need: critical temperature (Tc ), critical
pressure (Pc ), and
acentric factor ().
These are tabulated in
Appendix C-1.
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310 Fundamentals of Chemical Engineering Thermodynamics
Peng and Robinson’s method of relating a to temperature was also substantially the
same as Soave’s:
5
a
ac
(7.50)
although they used the symbol k where Soave used the symbol m for
5 [ 1 1 ( 1 2 T 0.5
) ]2
r
(7.51)
While k is related to the acentric factor in a manner analogous to Soave’s m, the
specific numerical values of the coefficients in Equation 7.52 are different from those
in Equation 7.46. Thus,
5 0.37464 1 1.54226 2 0.2699322
(7.52)
To summarize:
Conceptually, the van der Waals constants a and b can be computed if the critical
point (Tc and Pc) is known; no additional information is required.
In the Soave and Peng-Robinson equations, because a(T ) is a function of temperature rather than a constant, one needs an additional piece of information
beyond the critical point (Tc , Pc) in order to complete the equation of state. This
additional parameter is the acentric factor , which is described further in the
next section.
7.2.6 vapor Pressure curves and the acentric Factor
This section has examined P-V-T behavior of real compounds in the liquid and
vapor phases. Figure 7-3 illustrated the P-V-T data for water at a few temperatures.
Section 7.2.2 through 7.2.4 discussed how to model P-V-T behavior with cubic equations. To this point, we have examined single compounds—methane in Section 7.1,
pentane in Example 7-3, and water in Section 7.2.1 and in Example 7-4. In this section, we begin to explore similarities and connections in the behaviors of a variety
of compounds.
Qualitatively, essentially all chemical compounds exhibit the behavior illustrated
in Figure 7-3 and Section 7.2.1, although the specific numbers (vapor pressures, liquid
and vapor molar volumes, etc.) are very different from one compound to the next.
Among the key thermodynamic properties of a chemical are the vapor pressures
that correspond to each temperature; in Section 7.2.4, for example, we outlined the
importance of vapor pressure in interpreting the three solutions of a cubic equation
of state. It is instructive to plot the log of the reduced vapor pressure vs. the reciprocal of reduced temperature (log10 Prsat vs. 1yTr), as in Figure 7-8.
Notice that, for all of the compounds in Figure 7-8, the plot is essentially linear, although the slopes of the lines are different. The acentric factor, in effect, is
used to model these vapor pressure curves as straight lines. We know from algebra
that it takes two data points to define a straight line. However, all vapor pressure
curves end at the critical point. Thus, (1yTr = 1, log10 Prsat = 0) is one point on all
of the curves in Figure 7-8, and we only need one additional point to define each
straight line.
The acentric factor essentially provides a second point on the line; it is derived
from the value of the reduced vapor pressure that corresponds to Tr = 0.7. In fact,
Figure 7-8 is adapted from the first of a pair of publications in which Pitzer and coworkers introduced the acentric factor (Pitzer, 1955).
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0
20.5
log10
P sat
Pc
CH4
21.0
C (CH3)4
21.5
1.0
1.1
1.2
1.3
1.4
T 21
r
1.5
Based on Pitzer, K. “The Volumetric and Thermodynamic Properties of Fluids. I. Theoretical
Basis and Virial Coefficients,” Journal of the American Chemical Society, 1955.
C H A P T E R 7 Equations of State (EOS)
Figure 7‑8 log10 (Pr) vs. 1/Tr for five hydrocarbon compounds. The
dashed curves represent propane, n-pentane, and n-heptane from top
to bottom.
The value of log10 (Prsat) at Tr = 0.7 falls between 21 and 22 for the vast majority
of compounds, so the definition given in Section 7.2.4
1PP 2 *
sat
5 21 2 log 10
c
Tr 5 0.7
which is convenient because it produces values of that typically fall between 0
and 1. It is called the “acentric” factor because ∙ 0 for the spherical noble gases
(argon, neon, xenon) and = 0.011 for the spherically symmetrical methane, while
nearly all compounds are spherically asymmetrical and have significantly greater
than zero (Poling, 2001).
Why use Tr = 0.7 specifically in the definition of ? This is somewhat arbitrary in
that mathematically, if the log10 (Prsat) vs. 1yTr plot really is a straight line, any point
on the straight line would define the line as well as any other point. That said, a point
close to the critical point (say Tr = 0.97) would be a bad choice, since log10 (Prsat) vs.
1/Tr for real compounds is not literally a straight line, and we can better model the
entire vapor pressure curve by using two points that are farther apart (Tr = 0.7 and
Tr = 1) to define a line. This reasoning perhaps suggests a lower reduced temperature like Tr = 0.1 would be even better. However, our goal with the acentric factor
is to model the conditions at which liquid–vapor equilibrium occurs. If we choose a
Tr that is too low, we fall below the triple point, and there is no liquid phase at all.
Thus, when Pitzer and co-authors introduced the acentric factor, they chose Tr = 0.7,
because it was well away from the critical point but, in almost all cases, above the
triple point (Pitzer et al., 1955).
Chapter 6 showed that beyond P, T, and V, other properties of a fluid such as H,
S, U, and G can be quantified through an equation of state that accurately models
the fluid. Chapter 8 shows that vapor pressures and boiling temperatures can also be
quantified using an equation of state. Recall that our goal is developing equations of
state that are capable of modeling liquids, vapors, vapor–liquid equilibrium, and supercritical fluids—all with a single equation. Vapor pressures are, thus, a fundamental
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311
312 Fundamentals of Chemical Engineering Thermodynamics
aspect of what we are trying to model, and this is what motivated the innovation of
incorporating some vapor pressure data (in the form of the acentric factor) directly
into the cubic equations of state.
7.2.7 summary of cubic equations of state
The van der Waals equation can be written as
P5
RT
a
2
V 2 b V2
Z5
or
V
V2b
2
a
RTV
with
2
a5
2
27 R T c
64 Pc
b5
and
RTc
8 Pc
The Soave equation can be written as
P5
RT
a
2
V 2 b VsV 1 bd
or
Z5
V
V2b
2
1RTa 2 1V 11 b2
with
a 5 ac
ac 5 0.42747R2
T 2c
2
5 [ 1 1 m ( 1 2 T 0.5
)]
r
Pc
m 5 0.480 1 1.574 2 0.1762
b 5 0.08664R
Tc
Pc
The Peng-Robinson equation can be written as
P5
1 23VsV 1 bd 11 bsV 2 bd4
V
aV
RT
a
2
or Z 5
2
V 2 b VsV 1 bd 1 bsV 2 bd
V2b
RT
with
a 5 ac
ac 5 0.45724R2
T 2c
2
5 [ 1 1 ( 1 2 T 0.5
)]
r
Pc
5 0.37464 1 1.54226 2 0.2699322
b 5 0.07780R
Tc
Pc
7.2.8 residual Properties from cubic equations of state
In Section 6.3.6, we solved problems involving residual functions computed using
the van der Walls EOS. We derived Equation 6.157 (Example 6-9 Revisited), as an
expression for the residual molar enthalpy:
HR 5 RTsZ 2 1d 2
a
V
and Equation 6.168 (Example 6-10), for the residual molar internal energy:
UR 5
2a
V
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C H A P T E R 7 Equations of State (EOS)
313
An analogous expression can be found for entropy, beginning from the van der
Waals equation:
RT
a
P5
2
V 2 b V2
The partial derivative with respect to T is
R
5
1−P
2
−T
V2b
V
The molar entropy departure function, in general, is described by Equation 6.145 as
S 2 Sig 5 SR 5 R ln sZd 1
#
T5T,V5V
T5T, V5`
R
2 4 dV
31−P
2
−T
V
V
For the van der Waals equation, this becomes
SR 5 R ln sZd 1
(7.53)
T5T, V5`
3V R2 b 2 VR4 dV
#
3V 21 b 2 V1 4 dV
(7.54)
#
SR 5 R ln sZd 1 R
T5T,V5V
T5T,V5V
T5T, V5`
This integrates to
SR 5 R ln sZd 1 R fln sV 2 bd 2 ln sVdguV5V
V5`
3 1 V 24 *
SR 5 R ln sZd 1 R ln
V2b
(7.55)
V 5V
V5`
(7.56)
At the V = ∞ limit, b is insignificant compared to V, so the ln term is zero (ln 1). Thus,
the result is
V2b
SR 5 R ln sZd 1 R ln
(7.57)
V
1
2
When V 5 ∞, Z goes
to 1, so ln(Z) 5 0
Analogous derivations can be performed to obtain residual properties for any equation of state, including the cubic equations introduced in this chapter. For the PengRobinson equation, we have
UR
RT
HR
RT
SR
R
Z 1 s1 1
A
51BÏ8
211 1 Ï 2 ln 3Z 1 s1 2 Ï2dB
Ï2dB 46
kÏTr
52
5 sZ 2 1d 2
Z 1 s1 1
A
51BÏ8
211 1 Ï 2 ln 3Z 1 s1 2 Ï2dB
Ï2dB 46
5 ln sZ 2 Bd 2
ÏTr
51
A
BÏ8
21 Ï 2 ln 3Z 1 s1 2 Ï2dB
Ï2dB 46
ÏTr
Z 1 s1 1
(7.58)
(7.59)
(7.60)
in which:
A5
aP
R2 T 2
bP
RT
The use of these expressions is illustrated in Example 7-5.
B5
(7.61)
(7.62)
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314 Fundamentals of Chemical Engineering Thermodynamics
exaMPLe 7-5
cOMPuting resiDuaLs using Peng-rObinsOn eOs
Methane enters a turbine at T = 600 K and P = 10 bar, and leaves at T = 400 K and
P = 2 bar. Use the Peng-Robinson equation to determine the work produced for each
mole of gas.
SOLUTION:
Step 1 Compare to Example 6-9
This problem is identical to Example 6-9, except we are using the Peng-Robinson equation instead of the van der Waals equation. As in Example 6-9, the work will be calculated using the energy balance and residual properties where
·
WS
5 H 2 2 H 1 5 H R2 1 ( H ig2 2 H ig1 ) 2 H R1
n·
The change in molar enthalpy for the ideal gas state does not depend upon the equation
of state used to model the gas; it depends only upon the ideal gas heat capacity. Thus the
value computed in step 3 of Example 6-9 is again valid as
H ig2 2 H ig1 5 29289
J
mol
but the residual properties must be computed using the Peng-Robinson equation.
The critical point of
methane (Tc = 190.56,
Pc = 45.99 bar) and
the acentric factor
( = 0.011) are available in Appendix C.
Step 2 Fit parameters to the Peng-Robinson equation
The Peng-Robinson b parameter is calculated using Equation 7.48:
b 5 0.07780R
Tc
Pc
1
5 s0.07780d 83.14
bar ? cm3
mol ? K
190.56 K
5 26.80
2145.99
bar 2
mol
cm3
The parameter a is dependent upon temperature and is determined from Equations 7.49
through 7.52. Find ac:
T 2c
bar ? cm3 2 s190.56 Kd2
ac 5 0.45724R2
5 0.45724 83.14
(7.63)
Pc
mol ? K
45.99 bar
1
5 2.496 3 106
2
bar ? cm6
mol2
The parameter k is computed from the acentric factor where
k 5 0.37464 1 1.54226 2 0.2699322 5 0.37464 1 1.54226s0.011d 2 0.269932s0.011d2
k 5 0.3916
and is used to find , which is temperature-dependent. At 600 K,
5
600 K
5 0.4855
3 1190.56
K 2 46
5 [ 1 1 x 1 2 T 0.5
c ] 2 5 1 1 s0.3916d 1 2
r
0.5
2
(7.64)
Finally, a is determined using the results of Equations 7.64 and 7.63 for
a 5 ac 5 1.211 3 106
bar ? cm6
mol2
(7.65)
Applying the same calculations at T = 400 K produces a = 1.695 3 106 bar · cm6/mol2.
Step 3 Solve Peng-Robinson equation of state
The Peng-Robinson equation is
P5
RT
a
2
V 2 b VsV 1 bd 1 bsV 2 bd
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C H A P T E R 7 Equations of State (EOS)
and can now be solved for V. Both the inlet temperature (T1 = 600 K) and the outlet
temperature (T2 = 400 K) are above the critical temperature, so we only expect one real
solution for V. The results are summarized here.
T (K)
P (bar)
a (bar · cm6/mol2)
b (cm3/mol)
V (cm3/mol)
Z
Inlet
600
10
1.211 3 106
26.80
4990
1.0006
Outlet
400
2
1.695 3 106
26.80
16,590
0.999
The values of Z are both very close to 1 (even more so than when we were using the van
der Waals equation in Example 6-9).
Step 4 Calculate residual properties
For the fluid entering the turbine, A is computed from Equation 7.61 as
11.211 3 10 barmol? cm 2s10 bard
aP
A5
5
5 0.00487
RT
bar ? cm
183.14 mol ? K 2 s600 Kd
6
6
2
2
2
3 2
2
From Equation 7.62, we have
cm
26.80
s10 bard
1
mol 2
bP
B5
5 0.00537
5
RT
bar ? cm
83.14
Kd
s600
1 mol ? K 2
3
3
Everything required to compute the residual molar enthalpy from Equation 7.59 is now known
Z 1 s1 1 Ï2dB
A
11
ln 3
5
1
21
2
RT
BÏ8
Z 1 s1 2 Ï2dB 46
Ï
600 K
0.3916s
H
190.56 K
0.00487
5 s1.0006 2 1d 2 1
11 1 Ï0.4855 2
RT
0.00537Ï8 2
H R1
kÏTr
5 sZ 2 1d 2
5
R
1
3 ln
6
1.0006 1 s1 1
5 20.000907
31.0006 1 s1 2 Ï2ds0.00537d
Ï2ds0.00537d 4
1
H R1 5 s20.000907d 8.314
2
J
J
s600 Kd 5 245.2
mol K
mol
And for the fluid exiting the turbine, we have
H R2
RT
5 s0.999 2 1d 2
3 ln
5
0.00309
10.00162Ï8
2 11 1
0.999 1 s1 1
s190.56 K 2
0.3916
400 K
Ï0.6794
6
Ï2ds0.00162d
30.999 1 s1 2 Ï2ds0.00162d
4 5 20.00521
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
315
316 Fundamentals of Chemical Engineering Thermodynamics
1
H R2 5 s20.000521d 8.314
2
J
J
s400 Kd 5 220.7
mol K
mol
·
WS
J
J
J
J
R
ig
ig
R
·n 5 H 2 1 x H 2 2 H 1 c 2 H 1 5 220.7 mol 1 29289 mol 2 245.2 mol 5 29265 mol
1
2 1
2 1
2
The residuals computed in Example 7-5 are very similar to those calculated in
Example 6-9 and are small compared to the ideal gas change in enthalpy. Methane is
a very light and spherically symmetrical compound, so even at 10 bar, it is reasonably
approximated by the ideal gas law in this case. The Motivational Examples in Section 7.1, however, showed